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BRAKE PROBLEM - Pedal gets harder, tighter, locks


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1937 Buick with hydraulic brakes. - (also posted in Buick Section)

We completely rebuilt the brake system:

Master cyl kit

4 new wheel cyl kits

all 3 new flex brake hoses (Each front wheel, and from chassis to rear axle).

Drove the car on tour - after several miles, rear brakes and wheels got hot, so we let them cool - rear brakes were locked so I bled a small amount of brake fluid from a rear-wheel cylinder -- fluid squirted out under pressure - then subsided. Car drove OK for several more miles - then same thing.

I thought that the rear flex hose was collapsing internally, acting like a check valve - not letting fluid return, building pressure.

At the conclusion of the tour we replaced the rear hose and both front hoses.

The problem continued, so we replaced all of the steel lines as well as the hoses, rebuilt the master cylinder again, and replaced all 4 wheel cylinders with new. The problem continued as before.

We placed the car on 4 jackstands, and learned that, while we thought it was only the rear brakes, that was because they tended to lock and heat-up quicker. Really all 4 wheels brakes were affected.

Re-examination of the master cylinder suggested that maybe the piston was not returning enough to clear the return passage / internal bleed. I checked with a Buick expert, replaced the internal spring, and added a second spring which I managed to wind concentric with the first to increase return pressure on the piston, hoping to ensure that it cleared the passage.

It still happens, and after driving some distance, when I feel that the brake pedal is very high, tight, solid -- or the car will not rolll slightly at a stop, or notice a drag - it is time to stop at a level location, crawl under the car with a wrench, bleed the residual pressure from the brake system, wipe the dust from my shirt and jeans, and try to make it to the lunch stop.

Honest, guys and gals, I thought I understood these brake systems. Having replaced the flex lines twice, rebuilt / replaced all wheel cylinders and master cylinder twice, replaced the steel lines with the proper diameter lines, replaced all brake hardware and return springs with new parts, improved the return spring in the master cylinder ----

It has me stumped -----

I love driving this essentially original car, but would prefer not to have to

---- "Get out and get under" every 20-35 miles and shoot an ounce or two of brake fluid at the pavement. If the EPA is listening, I catch it in a coffee can for proper disposal.

I would be grateful for your advice, above and beyond what I've learned over the years -- I know I'm missing something.

Edited by Marty Roth (see edit history)
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The master cylinder piston might not be returning back all the way preventing fluid from returning to the Master cylinder and thus causing the shoes to not release. Look down into the master Cyl reservoir to see if the piston moves freely and returns completely when the pedal is released. You will see 2 small holes down at the bottom of the reservoir-look at the bigger 1/8"one and you will be able to see the piston move. You also need to make sure the brake pedal has 1/2" of free play before the pushrod pushes on the M/cyl piston. This assures the piston can fully return.

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The master cylinder piston might not be returning back all the way preventing fluid from returning to the Master cylinder and thus causing the shoes to not release. Look down into the master Cyl reservoir to see if the piston moves freely and returns completely when the pedal is released. You will see 2 small holes down at the bottom of the reservoir-look at the bigger 1/8"one and you will be able to see the piston move. You also need to make sure the brake pedal has 1/2" of free play before the pushrod pushes on the M/cyl piston. This assures the piston can fully return.

Ditto what he wrote. On many years of Chrysler products the rod from the pedal to the master cylinder is adjustable and if adjusted too long will result in exactly this problem. Physics are the same even if the brand is different, so it sure sounds like the piston is not returning past the relief port or the relief port is blocked.

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Ditto x 2 ! Classic symptoms of fluid not be able to return to the master cylinder.

Generally there should be between 1/8" to 1/4" free-play between the master-cylinder push-rod and the piston itself, with the brake-pedal against its mechanical stop.

When you say the system was "completely rebuilt", was the master cylinder sleeved to repair a pitted bore, and if so, did the jobber bore a hole for the "compensating port" (the tiny hole in front of the supply port)? If not, make sure that said tiny hole is not blocked, sometimes corrosion and/ or honing the MC bore can close it up.

Good luck with it Marty; '37-'38 Buicks are one of my favorites !

Frank McMullen

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We took a look at adjusting the free play of the rod going to the master cylinder this morning.

It appears that the free play did not adjust -- only lowered the height of the brake pedal in relation to the clutch pedal -- will try more tomorrow

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Marty, When I was a 15 year old kid back in the dark ages, I put motor oil in the hydraulic brake system of my '35 Pontiac. Duh. I seem to remember It caused the valve at the front of the master cylinder to swell and malfunction, and not permit pressure to subside when the pedal was let up. It was more than just embarassing to have to get out and bleed off fluid right in the middle of Wash. D. C. traffic. (Also a miracle I/m still on this side of the sod).

It may also be possible in some configurations for the valve to have been installed backwards. Maybe worth checking. Dave

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Was the master cylinder piston replaced in the rebuild ? If so, perhaps the piston is slightly different and not allowing the fluid to return through the tiny compensating port...

The larger supply port is approximately 1/8" to 5/32" in diameter... the tiny compensating port is probably 1/16" or smaller... if it is blocked, it is impossible for fluid to return to the reservoir.

The compensating port is usually 1/4" to 1/2" towards the outlet end of the Master cylinder from the supply port.

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I had a similar problem (symptoms, anyway) with my '47 Dodge brakes.

It turned out to be an improperly-assembled aftermarket cylinder. I had to take it apart before installation to out some machining residue and, while I thought I had put it back together correctly, one of the rubber washers was eventually found to be in the wrong spot.

First photo shows how I had it back together initially with the red arrow pointing to the offending washer. The washer kept the piston cup from clearing the compensating port.

Second photo shows the parts in their proper position, with the washer in question in front of the check valve where it belongs.

post-59237-143138721547_thumb.jpg

post-59237-14313872155_thumb.jpg

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Marty-- I had very similiar problems with my '36 80C and found a bad hose from the rear of the master cylinder to the front/rear distibution block. The hose had deteriorated internally and was working as a check vavle. New hose, no more problem.--Bob

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Pulled it apart and checked-

Both passages are clear-

The piston seems correct, and allows for the bleed passage-

The brake shoes and drums do not seem to be the problem-

The problem now comes on within less than a mile of driving-

All flex hoses , and all hard steel lines have been replaced in the last 18 months since the problem started-

We will next take off the flex hose right behind the master cylinder to see if this "new" hose may have a problem, by forcing compressed air backward to see if it will act as a check-valve

Tearing out what is left of my hair

Thanks for all of your responses

Marty

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Watching this saga with interest and experiencing your frustration, but cant help feeling that the root cause has all the hallmarks of master cylinder secondary / compensating ports not being uncovered as brake pedal is released.

Are you able to view these ports with cap removed whilst mc is installed; at the risk of teaching how to suck eggs, you should be able to run a fine wire through the compensating port with brakes relaxed.

Next apply and release brakes and observe fluid returning into mc through secondary port as brake pedal travels back to rest, followed by fluid transfer into mc through compensating port as wheel cylinders retract.

Not knowing the precise details of your mc kit seal construction, it is also possible that a check valve is incorrectly installed or a misplaced washer is blocking holes in the primary piston head.

As to your collapsed line theory, I would expect that this only affects the one wheel not all four.

Please dont be offended if I am overstating the obvious.

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The problem almost HAS to be in the master cylinder. How were the brakes before you rebuilt the system? Did the same pressure build-up exist before? If so, the problem lies in something that still wasn't addressed by the rebuild. If not, it comes down to improper assembly, wrong or defective parts, or incorrect adjustment of linkage.

Was the front (output) end of the cylinder dismantled and parts replaced there? In the right sequence? The valve there must be able to allow fluid to return into the cylinder when the pedal is let up. Second. are you sure the cylinder piston retreats far enough back for the rubber cup it pushes to not obstruct the small "compensating port" in the bottom of the reservoir? If it doesn't, compare the new piston's length to the old piston's to make sure it isn't too long. Test the port opening with a small wire, don't force, it should go right through. If the piston is correct and the port is still obstructed then it is a problem with something not allowing full retreat of the piston. Possibly a very rough boring of the cylinder? Not likely. Since ALL the wheels lock up, about the only other possible suspect would be an obstruction in the line from the output end of the master cylinder to the distribution block where fluid is directed to wheel cylinders.

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I have had this happen three times. In all cases, the brakes lines were getting hot. One car the heater hose was lying on the brake line and the other car a new brake line was run too close to the exhaust. Third car was a custom build and again the proportioning valve was installed too close to the header. All were heating up and expanding to a point the brakes lights were coming on and of course brakes locked...all three cars.

I am not a mechanic, just a parts changer. :-)

Edited by Tom Laferriere (see edit history)
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We slightly enlarged the bleed passage in the master cylinder -- no improvement !

Briefly under the car -- steel brake lines do not appear exceptionally close to exhaust system - the junction block and brake light switch are 3+ inches above the muffler -- brake line is 2 inches above front edge of muffler. I think this would be enough clearance, but will experiment with insulating material.

Still looking for a dramatic cause....

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3" my be too close, especially if you have stainless steel brake lines OR SS exhaust system. The size of the bleeder hole is not as important as making sure the plunger returns all the way back past the bleed hole. You can also get excess heat build up if the brake shoes are improperly adjusted. If the brakes are dragging, heat will build up in the wheel cylinders and cause an expansion problem too.

Are you using silicon fluid?

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Brakes are not dragging, at least prior to driving, wheels turn freely.

Not using stainless steel brake lines nor exhaust.

Not using silicone brake fluid - only DOT-3.

Will next try insulating layer between front edge of muffler and steel brake line / brakelight switch.

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If the issue is heat causing the fluid to boil in the lines, then the pressurized brake fluid should be able to escape back through the MC, via the check valves and compensating port. If anything, I would expect a spongy pedal in this case, due to vaporized fluid in the line....

I still believe the issue is some sort of blockage/ restriction preventing fluid from returning to the MC.

Marty - are you able to replicate the pump-up and brake-lock-up in the shop with the car parked and engine not running ( ie: removing the exhaust heat issue from the equation.)? Not fast-pumping the brake pedal, but simulating how you would apply the brakes as in your previous trips where the pedal / brakes locked-up...

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If the issue is heat causing the fluid to boil in the lines, then the pressurized brake fluid should be able to escape back through the MC, via the check valves and compensating port. If anything, I would expect a spongy pedal in this case, due to vaporized fluid in the line....

I still believe the issue is some sort of blockage/ restriction preventing fluid from returning to the MC.

Marty - are you able to replicate the pump-up and brake-lock-up in the shop with the car parked and engine not running ( ie: removing the exhaust heat issue from the equation.)? Not fast-pumping the brake pedal, but simulating how you would apply the brakes as in your previous trips where the pedal / brakes locked-up...

I too still think it is a problem with some blockage or restriction preventing fluid returning to the MC and think the MC is the most likely culprit. However I am not sure that applying the brakes while stationary will do the trick as it will not generate heat in the wheel cylinders as it would during normal driving: Just moving a brake shoe against a stationary drum doesn't heat things up.

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Marty,

It is a lot easier sitting here giving you advice than it is doing the repair, but you need a diagnostic plan. Hope this helps.

Have you tried releasing the pressure by loosening the brake line at the master cylinder? If this eliminates the symptoms it is a good chance that you do not have a a blockage down stream. It sounds like you have a compensating port issue. A heavy brake padel, incorrect or weak return spring, or incorrect or misadjusted push rod could cause this problem.

You could try using an old fashion hand pump oil can filled with clean brake fluid connected with a tight fitting rubber vacuum hose to the open rear wheel cylinder bleeder. This forces fluid up from the wheel cylinder to the master cylinder it should flow back to the master cylinder easily, if the compensating port is open. If it does not flow back to the MC work your way back to the wheel cylinder from the MC following the brake line and crack each fitting as you go once you release the pressure you have found your blockage. My knowlege of hot brake fluid is it causes soft pedal. I have copied a passage from one of the bake parts suppliers.

" Boiling produces gas bubbles within any boiling fluid. Gas is compressible so boiling brake fluid leads to a “soft” brake pedal with long travel. In extreme cases overheated brake fluid necessitates “pumping the brake pedal” in order to get a pedal at all."

This is link to complete letter.

http://www.centricparts.com/files/Centric%20White%20Paper%20D1-Brake%20Fluid%201A.pdf

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Agree with the above hot fluid will cause loss of pedal as fluid expands and in extreme cases boils, we are talking race car brake loads now.

Also agree with the problem being in the mc; can you possibly beg / borrow / steal another mc and swap it out to prove this one way or the other.

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The next time it happens, disconnect the pedal push rod at the m/c and see if that releases the pressure. If not, make sure the m/c piston is in the full return position and push it into position if necessary.

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I too still think it is a problem with some blockage or restriction preventing fluid returning to the MC and think the MC is the most likely culprit. However I am not sure that applying the brakes while stationary will do the trick as it will not generate heat in the wheel cylinders as it would during normal driving: Just moving a brake shoe against a stationary drum doesn't heat things up.

Tod, et al,

Some folks were offering the suggestion the heating of the lines / fluid system from engine exhaust / brake heat might be the culprit, that is why I suggested doing a static test with the vehicle parked, not running: ie, no heat, to confirm or eliminate heat as a variable.

If the system were still to "pump-up and lock" with a cold car sitting in the driveway, then it would stand to reason that exhaust heat / brake heat are not the root cause - which takes us back to check valves, compensating ports, pedal push-rod adjustment, etc.

I am still of the opinion that if all the lines and flex hoses are clear, and the check valve(s), springs, pistons, compensating ports are clear and in proper adjustment, then if the plumbing were to get hot enough for the brake fluid to exapand, boil / vaporize, any pressure caused by that action would push the fluid / vapor back towards the reservoir, and if anything, cause a low and/spongy pedal.

The fact that all four wheels are being affected suggests to me that the problem in in the Master Cylinder or the main system line to where ever it branches-off to the various wheels...

I am intrigued by the presence of a flex-hose between the MC and rest of the system, which leads to believe that the MC is mounted of the transmission or bell housing, or other driveline component that will be subject to torque-reaction...

Just clarifying where I'm coming-from at this point...

I think I would be looking for another Master Cylinder to try at this point...

:confused:

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Sunday I will get back to the Buick, and will exercise the brake system without starting the car. I will try to replicate the problem without generating any engine/exhaust heat in an attempt to rule out one factor at a time, and will report back tomorrow on the results.

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Assuming the Buick is a running, driving car, what problems existed before overhauling the system? If the brakes didn't lock up before and nothing has been re-positioned or changed, you can stop worrying about lines getting too hot. Something else, not yet addressed, is the culprit. Since you have beaten the master cylinder aspect to death I assume, it must be one of those simple to overlook causes, like the return spring not pulling the pedal all the way back, causing the rubber piston cup to still block the port. Next time it drags pull up on the pedal to see if that relieves the pressure.

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The Buick is a running, driving car, but we don't know what problems existed prior to getting it.

It was a parade car for Mayor LaGuardia, and is believed to have served New York City for perhaps a dozen years, kept on long after the manufacturers no-longer produced convertible sedans. The big Buick then spent likely 50 years in a museum and a private collection, not driving any appreciable amount, if at all.

I test-drove the car in January 2009, made an offer to the Law firm handling the estate of the deceased owner, got agreement and the car in July, 2009, and immediately had the brake sysytem and parking brake cable, exhaust, and all fluids replaced. The car was driven short distances of perhaps a few miles by the friend who did the work, and exhibited no problems.

I retrieved the Buick, trailered it to the start of the 2010 Founders Tour, and after the first thirty miles or so, found that the rear brakes and wheels were hot, but not so the front. Cracking open the bleeder relieved the pressure, so we assumed that the rear hose from the chassis to the differential housing was acting as a check valve. That flex hose was replaced, along with all others, and the entire system was gone through again, including all steel lines which were run just as the originals had been.

All wheels turn freely, and even the breeze can spin them, but when the brake system builds pressure the front, as well as the rear brakes apply, so we have learned not to concentrate only on the rear (as we had first assummed).

The return spring pulls the pedal all the way back, and the pedal is at the top of its travel,. and cannot be lifted any higher when the problem occurs.

When I first get into the car I can depress the brake pedal 2 - 3 inches, but in short order the pedal becomes firm, and then solid at the top of its travel.

I did not get to the car Sunday, but will attempt to get there Monday, and will work the brakes multiple times, not "pumping", but will simulate as if the engine were running and I were driving in moderate traffic - then see if the pedal goes hard. This could be a way of eliminating the "heat" as a possible factor.

Thanks again for all of the advice and concern.

I look forward to your thoughts toward making this a dependable tourer.

Marty

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We removed the extra spring. They were wound together, not to extend the length, but to provide more force to return the piston.

Now with the relief passage opened slightly larger, we will try it out around Thanksgiving. I'll be on the road to AL, OH, and PA, and then back home, hopefully with the 1914 Buick.

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A correct Master Cylinder Rebuild Kit was ordered from Bob's Buick, and was professionally installed. Of course we cannot confirm that the cylinder was never replaced in the past, but the car was believed to be a 7,9xx mile original when I bought it in December of 2008. After retrieving the car in July, 2009 it was treated to tires and tubes, exhaust system, and brake work. It was next driven on AACA's Founders Tour in June of 2010 where the brake problem was noticed. It has had only sporadic use since that time as we are still trying to solve the problem.

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This may have been mentioned already, but is this the only master cylinder you've tried (even if you've rebuilt it)? If that's the case, I'd be sourcing another core and rebuilding it. Something has to be funky about that one.

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Well, ---- we went back to a single spring in the Master Cylinder after enlarging the relief port --- drove the car 2 miles and the pedal was getting progressively higher / harder.

Then backed out of driveway - BRAKE LIGHTS ON !!! ---- CAR NOT ROLLING !!!

Time to find another Master Cylinder -- anybody out there have one???

Is a 1937 Buick Century the same cylinder as a Roadmaster ??

I'D like to buy a NOS or properly remanufactured / rebuiltg '37 Roadmaster Master Cylinder

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