1969 Mustang V8 - Brake pedal stays down when engine gets hot

[Moodster]

My car originally came with drums all around but I upgraded to Kelsey-Hayes disc brakes on the front.  These were the same disc brakes as the factory used on early Mustangs that had this option.   I drove the car for several years without any issues but about two years ago we had our house renovated and I put the car in storage.  My problems with the brakes started after that.  First let me say that I've replaced just about everything in the brake system.  Two new master cylinders, new brake booster, new calipers, new front/back rubber lines, new lines going into the master cylinder and I've bled the system countless times. My proportioning valve and brake splitter block is about eight years old and I bought it from a reputable part supplier for a stock disc/drum 69 Mustang. Brakes work fantastic when I first drive the car however once the engine gets hot, the brake pedal stays down when I press it. A few clues:

 

1. I don't appear to have any brake fluid leaks. Vacuum booster is dry behind the master cylinder.  Discs and drums are dry, no visible movement in the level of fluid in the resevoir.
2. Bench bleeding the master cylinder shows no leaks and the piston return spring is very strong.
3. When the brake pedal sticks, I can easily pull the pedal back up with my foot and the car moves freely again.
4. I do have ceramic coated headers and the vacuum booster/master cylinder does get hot but I can still touch it.
5. If I turn the engine off when the brake pedal is stuck down, it will slowly release.
6. If I leave the engine on with the brake pedal stuck down - jacking the car up shows me that all four wheels have the brakes engaged.
7. My vacuum gauge shows 13-15 inches of vacuum to the booster when hot and 16-18 when cold.

8. On the latest master cylinder, I set the length of the pushrod to the master cylinder using a special tool.  I found that the pushrod was a lot shorter than it needed to be.  The rod going from the booster to the pedal is non-adjustable.  

9. 69 Mustangs didn't use a pedal return spring.

10.  I haven't replaced the rear drum wheel cylinders.  For all I know they are original however they don't leak and they do work.  I've been ruling them out since the problem appears to lock all the wheels. 

11.  If I start the engine and let it warm up in the driveway then I can repeatedly use the brakes without issue.  The vacuum does appear to be higher and steadier though than when I get back from driving and the brake pedal is staying depressed.

 

I suspect the problem is that the booster is not getting enough vacuum. However I don't understand why slightly low vacuum would cause this behavior. The booster is new but I had this same problem with the old booster.

 

I'm at a loss, I really need some insight into the problem so that I can just stop buying so many parts.  Thanks so much for any help.

 

David

[Jubilee]

Sounds like you have an all rubber component (valve, check valve, o ring) that’s shrinking and failing as it gets hot. Hard to believe in this day and age.

Use to see same problem in hydrovac assist on K model IHC  trucks in fifties-sixties.

[ABear]

What happens when you disconnect the vacuum line from the brake booster?

 

You could try disconnecting the vac line (block off the engine engine side of the vacuum line so you don't create a vacuum leak on the engine)  from the booster, start and run the engine until hot and then try applying brakes.

 

Brakes will work with no vacuum on booster as manual brakes with booster disconnected and if the pedal returns to normal position when engine is hot then issue must be with the vacuum booster.

 

If pedal does not return then you will need to look elsewhere..

 

Common issues with hanging brakes are rubber brake lines that have an internal obstruction or collapse which may not be noticeable from the outside of the rubber hose. This can cause issues with brake cylinders getting fluid when stepping on the brakes, but when you release the brake pedal, the brake cylinders cannot overcome the blockage in the rubber lines.

 

Rubber brake lines must be carefully handled, they are very easy to crimp and that crimp often will not be detectable from looking at the outside of the line. Rule number one on handling rubber lines is to never allow the brake cylinders to hang from the lines, always tie up the cylinders to prevent hanging, twisting or crimping or excessive bending the rubber lines.

[Moodster]

That’s a good idea - I’ll try unhooking the vacuum booster and see if the problem is still there.  I’m also going to try insulating the booster and master from the heat and seeing if that makes any difference.  Thanks everyone for the help.

[kramaton]

I had a 1979 Trans Am that the master cylinder went bad. I replaced it with a new aftermarket replacement and the car did the same thing you are describing.  The car was OK for a few miles then the brakes would start slowly binding up. When you let it sit for a couple hours they were released. Ended up being a bad new master cylinder. 

[Moodster]

I’ve been suspicious of the master cylinder all along but I’ve never been able to find a problem with it.  I bought a second new one just to be sure.  All three master cylinders have shown the same issue.  My original one worked for half a dozen years or more.  I think it could still be the culprit but maybe aggravated by some other condition.

[Ronnie]

23 hours ago, Moodster said:

9. 69 Mustangs didn't use a pedal return spring.

23 hours ago, Moodster said:

3. When the brake pedal sticks, I can easily pull the pedal back up with my foot and the car moves freely again.

Why not just put a return spring on it and stop beating your head against the wall?

[ABear]

1 hour ago, Ronnie said:

Why not just put a return spring on it and stop beating your head against the wall?

Because that spring was never needed before?

 

If you cut an artery, would you put a bandaid on it and hope to live?

 

Or would you rather get things fixed correctly..

 

Brakes are not a place to be lazy with, lives do depend on proper operation at all times..

 

Spring built into the booster should already be strong enough to return the brake pedal, adding an extra spring would make braking effort harder than it should be.

[Ronnie]

1 hour ago, ABear said:

Brakes are not a place to be lazy with, lives do depend on proper operation at all times.

If you think putting a return spring on a brake pedal is going to make the brakes dangerous you're wrong. The OP asked for something he could do so he could stop spending money on more parts. I offered a solution that should work. Adding a spring wasn't being lazy. It was a logical thing to do in light of all the other things he has already done. If you have a better solution, let's hear it. His car originally had drum brakes and they did have return springs... on the brake shoes. You're analogy of cutting an artery and your big overblown Band-Aid graphic were nonsense and contributed nothing to fixing his problem.

 

On 4/20/2024 at 9:16 PM, Moodster said:

I'm at a loss, I really need some insight into the problem so that I can just stop buying so many parts.  Thanks so much for any help.

 

[Stude17]

One thing I would look at is the brake pedal pivot to see that it is not binding.

[ABear]

10 hours ago, Ronnie said:

You're analogy of cutting an artery and your big overblown Band-Aid graphic were nonsense and contributed nothing to fixing his problem.

 

Actually, if you would have read my post above yours, I did mention HOW to troubleshoot AND diagnose the problem by disconnecting the engine vacuum. Removing the vacuum to the booster is not harmful, the system basically operates as non power assisted brakes which will require more pedal effort but this will isolate the engine and engine heat factor with the booster as part of the problem.  That is a far better way to figure out what is wrong rather than cover up a symptom that can lead to brake failure.

 

What have you "contributed"? Diagnosing? Nope, adding a spring doesn't diagnose and/or fix a dangerous problem, only hides a symptom. Glad you are not wrenching on my vehicles, I would have fired you for that.

 

Pretty much many if not all properly working vacuum booster systems work perfectly fine without the need for a external "pull up" spring, the booster if correct will already have an internal spring and as the OP mentioned is quit strong. There IS a reason why the pedal is not returning on it's own when hot, best to find out why and correct it properly. I suspect defective booster/check valves in the booster and disconnecting the vacuum disables the booster as a away to identify or eliminate where things have gone wrong.

 

OP has mentioned that the brakes LOCKUP AND STAY LOCKED UP when the pedal is down and stays down, that is a very severe safety hazard that needs proper diagnosing and not a mere fix of a symptom.

[Moodster]

Thanks guys, this is all good feedback and contributions - exactly what I needed.  It rained here all day yesterday however it looks nice today.  I'm going to do some of the tests mentioned and let you all know the results.  

[Moodster]

I disconnected the vacuum booster and did my typical drive into town, picked up some groceries and back.  Lots of different braking - mostly easy but a few quick, hard stops.  At first I thought I might not be able to do this since the pedal was so hard to press down however I got used to it.  The issue did not appear the entire time.  Then I got home, quickly hooked up the vacuum booster, started the car and the issue was not present at first.  I took it for a quick drive around my neighborhood and it showed up within a few hundred feet of my house.  One other thing I noticed is that the brakes do appear to be slightly dragging even when the brake booster is working.  That's because I found the car rolls so much easier and better without the booster hooked up. Am I safe to conclude the brake booster is bad?

 

 

[ABear]

Good!

 

This has pretty much eliminated everything from Master Cylinder, proportioning valve, brake hoses, wheel cylinders and even brake pads.

 

Booster is most likely bad or incorrect for your installation or possibly your rods and adjustments are still not correct length wise. Not sure if a OEM shop manual would have details on how to adjust correctly. Not to mention since this isn't a "stock" setup and is more of a mashup I am not sure how one would go about making the proper adjustments.. Could be too large of a booster or too long (deep) which may give longer throw as there are different sizes of boosters available.. I try to stick with as much keeping OEM stuff as it was from factory to avoid mashups like this..

[Grimy]

17 minutes ago, Moodster said:

Am I safe to conclude the brake booster is bad?

Isn't the booster--and associated plumbing and adjustments--the only variable in your test procedure?

[Moodster]

This is the new booster I used: https://www.npdlink.com/product/booster-assy-power-brake-midland-ross-style/215184?backurl=search%2Fproducts%3Fpage%3D1%26search_terms%3Dvacuum%2Bbooster%26top_parent%3D200001%26year%3D1969&year=1969

 

Here is a picture in the car: 

[Moodster]

Unfortunately there is no pushrod adjustment between the pedal and the booster.  When you set the pushrod length going to the master cylinder does it matter if you have vacuum being applied to the booster?  Would there be any harm in purposefully making the pushrod too short to verify that isn't the problem?  I used this tool when I changed the booster to be more accurate with the pushrod adjustments: 

 

https://www.npdlink.com/product/tool-power-brake-booster-rod-length-adjustment/222186?backurl=search%2Fproducts%3Fsearch_terms%3Dbooster%2Badjustment%2Btool%26top_parent%3D200001%26year%3D1969&year=1969

[ABear]

That booster looks too deep, typically the ones that would have been originally used would have been a shallow more flat pancake design.

 

Obviously, since your brakes worked fine without vacuum on the booster, I suspect shortening the rod from booster to MC might affect pedal position and even your braking. If you have a spare rod, you could try shortening the spare, but I am not sure it is going to work correctly even with shortening the rod..

 

I am thinking the booster has too much travel throw and possibly too strong and as things warm up the rubber diaphragm in the booster is easier to stretch..

 

OK, so looking again, your booster looks like a dual diaphragm.. Where the large vacuum line is connected, there is a small vacuum port which has a rubber cover on it.. I think that small port should be going somewhere other than being plugged off..

 

For the record, there are other sources that are cheaper for boosters and Midland-Ross style are being reproduced..

 

https://www.classicindustries.com/shop/1969/ford/mustang/parts/brakes/power-boosters/

 

Have to see if there is any vacuum diagrams around for dual diaphragm boosters, that smaller port would seem to need a place to go, can't imagine they stuck it on for no reason at all.

Edited April 22 by ABear (see edit history)

[joe_padavano]

Coming into this thread late, but your description sounds like a bad/leaking air admittance valve in the booster or a mis-adjusted pushrod between the booster and the pedal. The booster is just a diaphragm in a vacuum canister. With the brake pedal released, there should be engine vacuum on both sides of the diaphragm, so zero net force. When you press on the pedal, it opens an air admittance valve in rear side of the diaphragm. This closes the connection from the rear of the diaphragm to engine vacuum and allows atmospheric pressure to enter behind the diaphragm. With vacuum in front and 14-ish PSI in back, there is a net force on the diaphragm that applies the brakes. Yes, there is also a fail-safe rod that applies the brakes directly from the pedal if the booster fails or is not connected.

 

When you release the pedal, the air admittance valve closes to atmosphere and re-opens the connection to engine vacuum. This returns the booster to the un-applied state. If there is any leakage through this valve, the backside vacuum level won't be equal to the front, so there will be a net force on the diaphragm, causing the pedal to go down and the brakes to drag. The fact that this is engine heat related suggests two possible causes. First is that the pushrod from pedal to booster is just SLIGHTLY long, such that when cold it allows the valve to close, but once heated from engine exhaust pipes below the booster, the rod expands just enough to cause the air admittance valve to leak. The other possibility is that the valve itself is defective and again closes fully when cold but leaks a little as parts heat up and expand.

Edited April 22 by joe_padavano (see edit history)

[Moodster]

Wow, that is one of the best explanations I've read for how a vacuum booster works.  I was beginning to think I would never understand what was going on.  

 

To test for the pushrod to the pedal being too long, do you think I could put a washer between the booster mounts and the firewall?  The pushrod is non-adjustable on that side so short of cutting it into and threading it with some rod, jam nuts, etc I don't have any way to change that length.

[ABear]

Joe, I was looking for a diagram like that!

 

OK, so it is possible that the rod is a bit too long, did find that there are adjustable rods, might want to look into that although the booster in question has a fixed length booster to pedal rod, not sure if that can be replaced..

 

Have seen that some boosters also come with a spacer, not sure if that was between booster and fire wall or booster and MC.. No real detail there. But if it is the booster to pedal rod being a bit long then in theory a spacer could be put between booster and firewall. Temporarily for testing purposes, yeah a washers could be placed between booster and firewall to see if there is any effect. If it corrects the problem then make a shim ( cut and fit sheet metal to make a shim or two) that fits the footprint of the booster to firewall).

 

I have a mid 70's Ford with factory disc brakes but they are non powered so I don't have an example to look at for the booster setup so seeing that diagram is helpful.

Edited April 22 by ABear (see edit history)

[Ronnie]

23 minutes ago, ABear said:

But if it is the booster to pedal rod being a bit long then in theory a spacer could be put between booster and firewall. Temporarily for testing purposes, yeah a washers could be placed between booster and firewall to see if there is any effect. If it corrects the problem then make a shim ( cut and fit sheet metal to make a shim or two) that fits the footprint of the booster to firewall).

What?? You think putting a return spring on the brake pedal is dangerous and a bandaid fix and now you think making modifications like you describe are OK? What happened to your theory that it worked before so it should work now without modifications? Putting shims between the firewall and the booster sounds like a bandaid fix to me. Your bandaids are no better than mine.

 

Remember what you said earlier or have you already forgotten it? "Brakes are not a place to be lazy with, lives do depend on proper operation at all times.."

 

 

[Moodster]

These are all just spitball ideas to try to determine the real problem and I appreciate everyone's input.  I looked into a return spring today and there is nowhere to put it.  The brake pedal bends around the steering column and that area is really tight.  If indeed the pushrod length is the problem then a replacement booster from the same supplier will likely perform the same.  However if it is the valve then maybe it is just a defect in this one booster.  These ideas help me to determine the right direction.  Thanks everyone for the help.

[Bhigdog]

I had a similar problem on my 57 buick. After much trouble shooting and gnashing of teeth I traced it to what joe p called the air admittance valve. A very small compression spring had lost some of it,s compressive force. After 3 years of trying everything in cluding a return spring, disassembling the unit and stretching that spring  about 1/4" longer cured the problem. May not be your problem then again it does sound like a vacuum problem......bob