Spongy Brakes

[gvgehrig]

I'm trying to diagnose a spongy brake problem on a '41 Ford four drum setup; master cylinder with a single 1" bore is mounted lower than the wheel cylinders. Lines are bled, drums turned, new shoes, new lines, new master cylinder and tested, wheel cylinders rebuilt - no leaks anywhere. I put in a 1 1/8" bore rebuilt master cylinder, but it did not help. Anchors and shoes are properly adjusted; even tried tightening the shoes while on the lift so they had no movement. On occasion, I can get a hard pedal on the lift and for one or two hard brakes, but then it goes spongy again. I cannot consistently recreate this condition. Hydraulic brakes seem simple enough. The master cylinder pumps fluid out of the ports; the shoe springs push fluid back into the master cylinder when pedal is released. There is no check valve on the master cylinder to resist back flow, but the system is sealed, so vacuum pressure should hold the fluid in the wheel cylinders. Wheel cylinders have an internal spring to keep the seals expanded. Wilwood makes a 10# pressure valve which is supposed to keep the fluid from draining back into the master cylinder. Why is this extra valve necessary? Is it required for all four cylinders? Also, when I bleed the brakes again after getting the spongy pedal, I see no air in the lines. If fluid was draining back to the MC, shouldn't there be air in the wheel cylinder? Your thoughts would greatly be appreciated. Thanks

[richard crow]

the correct master cyl has a check valve inside it if you are using some kind of master cyl. that don,t have one. the purpose of the check is to hold low pressure in the system other wise when the pedel is released air is drawn in through the wheel cylinders if you have the orignal plumbing there should be one line leaving the master cyl. you can install a 10 lb check valve there now tell me you have a duel master cyl. & i wasted my time

[gvgehrig]

Thanks. It is not a dual master cylinder. If there is supposed to be a check valve, that helps me understand a couple of things- the 1 1/8” master cylinder I tried was defective and I probably didn’t get it bleed properly. Also, why would the wilwood valve be necessary?

[Mart]

If it's not a dual, why not use a stock early Ford master cylinder? 1-1/16" and a valve built in.

If all the bleeding side has been dealt with effectively and there is a lot of "give" in the system, then it can be due to Mismatch of shoes against oversize drums. The shoes have to bend to make better contact.

[gvgehrig]

I’ll try the 1 1/8” MC. If I’m going to change MC, then I’d like to put in a dual. The question then, do I need the wilwood pressure valves since the MC is lower than the wheel cylinders?

[TJ]

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If you are going to change master cylinders why not use a stock one with the check valve?It's the correct one to use with your stock '41 brakes.

[Fordestes]

On drum brakes a residual check valve is necessary , the valve holds residual pressure on the brake shoe assembly to avoid having to build up volume / pressure to overcome the return springs and displacement of clearance area between the shoes and drum every time the brakes are applied thus allowing faster application of brakes also as Richard Crow said to help avoid air entering the system.

[gvgehrig]

I talked to Wilwood this morning. They believe even with the built in pressure valve, I need the 10# on the front and back each. I'm struggling with understanding how air "is" getting into the system since I don't see any air when I re-bleed the brakes. I have an original Ford 1" single bore MC on now. I believe it has the check valve because the fluid only flowed one way when I bench bled it. The 1 1/8" bored MC flowed both ways when I bench bled it, which is why I didn't believe they had a check valve. Also, the wheel cylinders have springs behind the valves to keep them from closing beyond the pressure of the shoe spring. I arched the shoes, so they match the drums. To test the anchor and shoe adjustment, I closed the shoes against the drums so there was no play -- still had soft pedals. It gets pretty hard very close to the floor, but it will not lock the wheels when driving and trying to stop. I'm willing to put on a new MC and the Wilwood valves, but I've tried so much without success. I'm hoping someone has the "fix" and can explain it so I'm reasonably sure that when I spend the additional money and time, it will work. Thanks

[Mart]

Original Ford master cylinder is 1-1/16". Is there something you're not telling us? Is this stock 41 Ford, with original type non self energising brakes?

What is the 1" cylinder? What is the 1-1/8" cylinder?

Why not use a stock cylinder? Why complicate it with aftermarket valves and suchlike?

Mart.

[JSeery]

If the brakes are spongy there is air in the system somewhere, brake fluid does not compress. The residual valve(s) are to maintain pressure in the wheel cylinders. There are a number of sites online that go into great detail on how brake systems work.

On your immediate problem, how are you bleeding the brakes? Have they been pressure bleed? A very small amount of air can cause a lot of problems.

[gvgehrig]

Thanks for the replies, here's some better info and an update. The MC is a Ford F-100 1961-1966. MAPA part # P36072. Just put it on the rack, and this is my result today (I'm not skilled enough to get the same results everyday, which is probably my biggest problem.) I can pinch off the back two wheels and get a hard pedal. I can pinch off the two front wheels and get a hard pedal. I can pinch off only the left front and get a 1/2 pedal (good). I can pinch off only the right front, and get spongy brakes. So what I see is that my left front wheel is MOST of my problem. I re-bled the cylinder -- no air. Air is not my problem. I closed the shoes so they can't move, no change. I get a sense of how much peddle is being used by each wheel, so as I add wheels to the system, the pedal goes further down. If I could adjust the left front wheel to be no worse than the right front or back wheels, problem would be solved. There is no fluid leaking anywhere. With the wheel pinched off and a hard peddle, I can hold the pedal down with pressure, and it does not go to the floor. Pumping the peddle does not change anything (i.e. no air in the system). There's nothing to a wheel cylinder -- two pistons, two seals, and a spring. What could be the cause of this? Thanks so much.

[JSeery]

Where did the wheel cylinders come from? Have you tried changing out the wheel cylinder you believe is the issue?

[Mart]

A 1" cylinder is a bit small for stock ford brakes.

[gvgehrig]

Cylinders are a 48-51 Ford 1 1/8" and 1" on the front and 1" on the rear. Yes, I purchased one new front cylinder, and I rebuilt the other three. Since I can pinch off the line, I believe the problem is from the wheel cylinder to drum -- no other place to look. Since I can manually lock the shoes, that only leaves the cylinder. So, I can pull the cylinder and inspect it, but what am I looking to discover? It's not leaking. Ford made a 1 1/8" bored MC. Would that help? In 1968, they made an 1 1/4" bored, twin cylinder. Maybe that would work better. The web sites don't provide specs, so I can't tell what will fit my set up. It appears that more fluid volume would solve the issue, but why is one wheel so far out of specs than the other three? Thanks

[Ggmac]

Is the pedal to master cyl rod adjusted ?

[cmbrucew]

Are you using DOT 5 ?


Bruce

[Mart]

You have 48-51 brakes on a 41?

What are these, F1?

Or is the car a 51?

[gvgehrig]

It's a highly modified Model A pickup; I call it my Johnny Cash car. I bought it like that. Some things were done great; some not so great, including the brakes. Here's some better data; today I'm able to get consistent results. I have three flex lines I can squeeze off: two in the front and one for both rear brakes. With all three lines pinched (no active wheel cylinders), the brake pedal travels towards the floor only 1/2". This is the "free play" in the system. The pedal will hold that position, there is no leakage in the MC. If I open the rear line, I get 4" of travel towards the floor. I do not have a way to close off only one of the rear wheels. If I close all wheels except the right front, I get 1" of travel. If I open the rear, keeping the right front open, I get 5" of travel. If I open the left front and close all other wheels, I get 1 1/2" of travel. If I open the left front and rear (closing only the right front), I get six inches of travel. This is at the floor. I can stop the car from rolling, but it will not lock the brakes. It appears to me, I need more fluid flow. If this is correct, the question then is just how much? And how do I get it? I have full travel on the MC piston with my pedal. If I could get each wheel working like the right front, I'd have complete stopping power at about 1/2 pedal. Thanks

[gvgehrig]

Correction, I'm getting 2.5" of travel with only the left front open. This is the cylinder with the most travel. (I know I haven't separated the back wheels, but combined they're 4". I'm using DOT 4 fluid. It's all new, been bled several time.

Just FYI, when I started working on the brakes, I had no idea what I was doing. That was a month ago, and it's like having been through a crash course in a trade school. Lots of mistakes, many, many questions. Lots of help from many people. Started out thinking my brakes just needed adjusting, but learned I haf many issues. Been correcting them one by one. It's been interesting, to say the least. I feel I'm almost there. Just need to cross the finish line. Thanks again.

[JSeery]

If it is one of the China made wheel cylinders they often do not have the bleed hole drilled correctly. That allows air to be trapped at the top of the cylinder.