54 Cad; 4W Disc Conversion; Soft Pedal Issue

[Dennis in CT]

So I am throwing this out there with the hope that one of you folks can help.

 

I recently ordered and installed 4 discs on my 54 Cad which I purchased through Caddy Daddy. No issues installing, but I no matter what I do, manually bleed or power bleed, I CANNOT get a hard pedal - instead it is squishy. There are no fluid or air leaks, and I have bench tested the Master several times - it seems fine. When I have plug the front and rear ports, pedal is hard a rock. I have tried isolating the issue by blocking just the rear port, then bleeding the front calipers (and vice versa) and I still get a squishy pedal. So it would appear that all things point to the master, but why does it work on the bench? I am thinking that maybe the bore size is wrong? I have mounted the new MC in the same exact spot as the old one, which means it is just below or sorta even with the calipers, so perhaps I need to install 2 lb residual valves? But would that really make for such a squishy pedal?

 

Hoping some brake genius is out there that can help - many thanks

[Matt Harwood]

Is the master undersized? Four-wheel discs move A LOT of fluid. Using a master from a car with drum/drum or disc/drum brakes probably won't be adequate. Is this a "kit" or something you created yourself? Is there still a booster for power brakes? What master cylinder and calipers are you using? So many variables that it's hard to figure out what's going on from what you're describing. This is also a common problem with disc brake "upgrades" that come in a kit. Installing the stuff is only the first step in a long, frustrating process because you're essentially re-engineering the entire braking system on a very big, heavy car. When they say "bolt on" I usually laugh in their faces. It never works that way.

 

Can you give us an idea of what the parts are and maybe we can point you in the right direction? Have you contacted the vendor who supplied the kit/parts? How did you select the parts? More details and maybe we can help.

[Bloo]

There are several things to pay attention to in a swap like this.

 

1) Pedal ratio. I believe it should be about 3:1 for power brakes, and 5:1 for manual brakes. This assumes a normal modern diaphragm-type brake booster. I got the numbers from "How to Make Your Car Handle" by Fred Puhn, but it has been decades since I read it and those numbers could be wrong. No matter. Anyone who wants to design or redesign a car needs to read that book (and probably everything Carroll Smith ever wrote). You might notice that in most cars where both manual and power brakes were offered, there is a removable panel on the firewall. It isn't just to get a different bolt pattern in the firewall. It allows the power booster to mount lower so that the pushrod can line up at the correct ratio.

 

2) Pedal Height and throw. The pedal (and brake booster if present) needs to be able to bottom the master cylinder with no fluid in it. Keep in mind the dual master cylinders usually used in disc brake conversions need more travel to bottom out than a typical single cylinder.

 

3) Master cylinder type. Master cylinders for drum brakes usually have "residual pressure valves" built in. After cup expanders for wheel cylinders came along, they weren't technically needed anymore, but the manufacturers kept putting them in, perhaps as a belt and suspenderss approach. So, out of a series of dual master cylinder, there will be three different types. A). 4 wheel drum with 2 residual pressure valves, B). Front disc/rear drum with one residual pressure valve, and C). 4 wheel disc with no residual pressure valves. A residual pressure valve will make disc brakes drag, so if you use a master cylinder from a an application with any drums you will have to pull the tube seats out and get those valves out of there.

 

4) Master cylinder bore. The hydraulic ratio between the calipers and the master cylinder bore affects pedal travel and sponginess. The smaller the master cylinder bore is, all else being equal, the longer the pedal travel will be and the easier to lock the wheels it will be, and the spongier the pedal will be. Keep in mind that one step bigger in bore size is a bigger change than one step smaller (pi R Squared).

 

5) Brake booster power vs hydraulic ratio. If you make the master cylinder bore too big in relation the caliper bore size, the stiffness at the master cylinder can require more force than the diaphragm in the brake booster can produce. The result? Brakes that stop nicely with your big toe in normal traffic, but wont lock the wheels, and need to be stood on to get enough braking in a panic stop.

 

5) The combination valve, or metering block, or proportioning valve. Cars with front disc/rear drum usually have a metering valve on the front brakes to keep them from locking before the rear drum springs have a chance to stretch. There may be a proportioning valve for the rear brakes as well. If any of this stuff exists it probably needs to go away in a four wheel disc setup. Four wheel drum systems usually set up the proportioning by altering the rear wheel cylinder bore size. While that should work on 4 wheel disc, you generally don't have the luxury of changing rear caliper bore size. You will have to do that with a proportioning valve, and since this is a custom application, it needs to be an adjustable one.

 

6) If you like a really hard stiff pedal (I do), consider getting some stainless brake hoses. They have a limited lifespan due to dirt grinding away at the teflon liner, but DOT rated ones exist today, and they should last a while. This eliminates most sponginess due to hose stretch. Do this last. It is a tiny change compared to everything else.

Edited July 20, 2018 by Bloo (see edit history)

[Dennis in CT]

First off, thank you both. Matt the calipers appear to be good quality - not sure of the source, but judging by the Grade 8 bolts and what not they seem domestic to me. I am working with the vendor but it has been frustrating to say the least.  Bloo I really appreciate the way you spelled it all out. I have been reading alot about this subject the last few days and you really condensed it into something I can digest. 

 

So they are going to send me a new master so we can eliminate a defective master from the equation, They are claiming the one I have has a bore size of  1-3/16", and their tech said that "should be more than enough". What I dont understand is how one determines what bore size is right for each application. So in my case, how do I verify 1-3/16" is enough? Im going to pull off the one I have this weekend and check the bore to see if this one is indeed that size. Maybe I'll get lucky and its the wrong size  - but I doubt that as this project has been cursed for the start. But hey, you dont learn unless things go sideways.

 

So i will circle back once I get the old master off and get the new one up in there - wont be till the end of the month. In the mean time I will try and figure out why my Graham wont go into gear.

Edited July 20, 2018 by Dennis in CT (see edit history)

[Bloo]

I don't know how to calculate it anymore or even estimate it. I have done it, but it was decades ago. I take it there is no booster? If so, pedal ratio and bore size might be more or less interchangeable. Decrease the pedal ratio or increase the master cylinder bore size for a harder shorter pedal.

 

Does it stop good even though it is spongy? Did you bleed at the master cylinder as well as the wheels? Are there any "high spots" in the lines that could trap air?

Edited July 20, 2018 by Bloo (see edit history)

[JACK M]

Is there a proportioning valve in this set up?

Residual valves?

My last couple of brake problems were a MC with to large of a bore.

Bigger bore equals less pressure.

My buddy that I am helping now has an OK pedal and stops straight, but says there is no way he could lock them up in a panic situation.

Last time I ran into this I went with a smaller bore MC and it helped a bunch.

Also the lines you use can affect pedal feel. Just another thought.

There are plastic lines on the market these days that result in a soft feeling pedal.

 

[W_Higgins]

When you did the conversion did you happen to switch from DOT 3 to DOT 5 (silicone) brake fluid?

[tripwire]

On 7/20/2018 at 1:06 AM, Dennis in CT said:

What I dont understand is how one determines what bore size is right for each application. So in my case, how do I verify 1-3/16" is enough?

Dennis,

Like everything now. Google your question.  There are a ton of sites discussing how to select the properly sized master cylinder.

 

Here's one: https://www.joesracing.com/rt-4172-master-cylinder-math.html

 

Wes in VT

[JACK M]

1 3/16" is pretty big.

Smaller dia = more pressure. Larger dia = more volume.

However these day of big boosters and small boosters or no boosters some practical application would be in order. (try different ones)

 

[Ovalrace25]

I really dont want to get into this--- Last SEVERAL projects were all huge nightmares! But here go a few  comments. The master cylinder IS lower than the calipers. You NEED residual check valves...  Going bigger on the master cylinder only gives you a pedal so hard you cant stop...  BLEEDING the system to get ALL the air out is a huge game to play!  The rear calipers that have the park brake lever (GM units) require a particular procedure to insure they are properly adjusted before you try to bleed the system.  DO NOT use silicone fluid! That in itself causes a "spongy/soft" pedal. 

Remember, the car stopped OK with stock drum brakes-- Adding front disc only, will help it more-- the rear brakes contribute the smaller percentage of stopping power, so what change them at all? Betcha could have skidded to a stop with the original system! It was the rear brakes skidding, not the front.. 

So if you want to stick with the 4-wheel disc and keep the master under the floor.. FINE-- Get the Electric Power Booster Master Cylinder kit and be done with all this! (do a google search) The car will stand on its nose when you touch the brakes. BUT only if you have some big rubber on the ground! if you have original tire size or reproductions, you will still-- Just Skid.   

  

 

[Frank DuVal]

5 hours ago, Ovalrace25 said:

DO NOT use silicone fluid! That in itself causes a "spongy/soft" pedal. 

Not if you are below 10,000 feet elevation......

 

I've been using DOT 5 since the early 80s in hundreds of vehicles. Never a problem with pedal stiffness. 

 

Of curse, I do not aerate the DOT 5 before I pour it in the master.....? And I do not pump the pedal while bleeding the system like a bass drummer for Metallica. I have seen one person do that, even moved the firewall he used so much force. ?

 

Now, are all four bleeders at the TOP of the calipers? If they are slightly down or worst case at the bottom of the calipers, no way can you bleed all the air out of a system. I see this in pictures of people doing conversions. No wonder they are posting problems.......?