Brakes-no brakes

[Kaiser2boy]

Pump up the brake brakes work fine,,,once.
Let foot off wait 10 seconds , pedal hit the floor.
Pump it up all is well for one more stop.
Replaced master cylinder, same problem.
Removed fill cap and watch whats going on in side.
Pumped it up ,I see some movment of fluid as I expected. As long as I hold the brake all is well.
I start to let up on the pedal and just as it reaches the top of the stroke I see a rush of fluid in the master cylinder fluid.
I do this over a few times and see that if I let the pedal all the way up I loose any pedal I had.
I look at the linkage for the brake and see what looks like an adjustment to make the master cylinder rod a bit longer or shorter. I make it a tiny bit longer, about 1/4 inch and the problem went away,,,,I think.
Anyone run into this ?

[gusbratz]

maybe a bad spot on one of the wheel cyl's. when you have pressure on it it is ok. when you let up and the springs pull the cyl back to gether it gets back into that pitted area? If that is the case then you should see the fluid leaking out the bottom of the drum. it must be going somewhere.

[OLD DODGE]

It sounds like your brake shoes need adjusted.

Disc vs drum

With the modern disc brakes your pad lies right on the rotor and doesn't require a lot of fluid volume to move the piston to apply the brakes.

With drum brakes you need more volume to move the pistons out of the wheel cylinders to make the shoes contact the drum. If one needs adjusting it will require more volume. Times four and your pedal will hit the floor but you can pump it up like you are doing.

[Kaiser2boy]

I bled the system, checked all the wheel cylinders and there is no leaks or sticking parts or pitted cylinders.
I even replaced all the hoses.
The brake fluid is not going away it is just flowing back and forth in the resivoir . If I let the pedal up too far I see a flush of fluid in the resivoir and then loose my pedal.

Like I said in the OP once I adjusted the master cylinder push rod so that the pedal did not travel so far all was well. If I let the pedal move one half of an inch farther up it seems the master cylinder looses its prime if that possible.

[Gsmith]

I went through the same scenario, brake adjustment is critical. The master cylinder needs to return all the way, and the rod should be adjusted so there is an ever so slight amount of free play. It sounds like you built in a bit of preload so to speak, which is wrong.

[Kaiser2boy]

I went through the same scenario, brake adjustment is critical. The master cylinder needs to return all the way, and the rod should be adjusted so there is an ever so slight amount of free play. It sounds like you built in a bit of preload so to speak, which is wrong.

I figured that.
I went back and re-did everything . One rear brake was off a bit but not much. Re-bled the system but I did not shorten the master cylinder rod.
I have about an inch of free play on the pedal and then good hard pedal.
I cant get it on the road right now because it has been raining non-stop for two days now and my driver side wiper is very lazy and as you know feeding gas will stop them. I tried just pushing the truck by hand in my garage to see if any wheels are to tight and it rolled a bit so it cant be too tight.
I will see after the Typhoon is over how it goes.

[Gsmith]

Just in case let me clarify that the free play should be at the rod not the pedal. In other words the rod should be adjusted so that it's just shy of putting any pressure on the master cylinder.

[Kaiser2boy]

Hmm,
When I do that I loose my pedal. I watched inside with the fill plug out and if I let up on the pedal very slowly as soon as it comes to rest with slack on the rod I see a rush of fluid in the reservoir and if I press the pedal it goes right to the floor.
It makes no sense at all.
If I open the rod so that it pushes the piston in only 3/16 of an inch it works fine. It just seems like it looses prime if I let slack in the piston.

The master cylinder has a place for a vent line that runs up to the aircleaner. The one I took off did not. I have not hooked up the vent line because It looks like it should have a shut off valve in the line and it does not so I left it pluged. I dont know if it should be pluged or vented. The manual shows a shut off valve mounted on the master cylinder. Should it be kept shut or open ?
Could that be the problem?
This is not the first master cylinder I have replaced but it is the most quirky by far.

[OLD DODGE]

as soon as it comes to rest with slack on the rod I see a rush of fluid in the reservoir and if I press the pedal it goes right to the floor.

That would be the return springs pulling the shoes away from the drum, pushing the pistons back in the wheel cylinders, forcing the fluid back through the master cylinder and into your reservoir.

It still sounds to me like your shoes need adjusted. You are using all your pedal volume trying to get the shoes out to contact the drum and you have nothing left to stop with unless you pump it. Either that or the master cylinder is too small but I still suspect you need to adjust the shoes AFTER you adjust the master cylinder push rod as Gsmith has described.

[Gsmith]

How are you adjusting the brakes? Upper and lower adjustment?

[Charles Talbert]

A proper shoe to drum clearance adjustment may well be your issue. I've done some very detailed post in the past here on how to do a proper adjustment in this type system. A search should bring that info up for you.

No vent in the reservoir will cause a vacuum to be drawn when pedal pressure is applied. It must have a vent for proper operation, it should never be plugged.

A failed residual check valve in the M/C will also cause this problem as you described. When functioning correctly, the check valve will hold slight residual pressure in the lines, hoses, and wheel cylinders when the pedal is released. When pedal pressure is applied at the next brake application, system fluid pressure will start to build immediately since all the components are already filled with fluid. If the check valve is not functioning correctly, all fluid volume will flow back to the reservoir when the pedal is released; thus requiring pumping action to refill the system again before pressure can begin to build. If shoe to drum clearance is correctly adjusted and there are no leaks in the system; the check valve is the most likely culprit causing what you have described. The rushing of fluid back into the reservoir just as pedal is fully released suggest strongly this check valve could be your issue. It should be working correctly in a new M/C for sure, however anything is possible, I have found check valves installed backwards in new M/C's. If you are purchasing either an NOS or rebuilt M/C as a replacement, you are just asking for issues. 100% new brake components are the only way to fly.

One other question; you stated that the wheel cylinders were free of pitting; how could you know that? The only way to tell is to remove them from the truck, disassemble, clean, and closely inspect.

[Kaiser2boy]

The check valve answer sounds like what I have. As I stated prior , at the very last split second before the pedal comes to a rest at the top I see a rush back into the master cylinder and then I need to pump it up again.
It is a brand new cylinder from Vintage power wagon.
It was a bit disapointing to see that it was made in Taiwan.
I hate to pull a brand new item apart with out any clue as to how the check valve should be set or just where it is.

The wheel cylinder look like new ones. I peeled back the rubbers and peeked in. There was no seepage or rust,,,that I could see. They work smoothly and the brakes shoes are in good shape.
The odd thing is that if I adjust the rod that goes into the mstcylinder so that it has tension on it and keeps the piston from comming fully out , the brakes work just fine. It is only a matter of baybe 3/16 of an inch in from full out. I took the truck for a ride and the breaks work fine. The breaks release just fine , no draging.

I called VPW and asked about the problem and they say "there is no way you can bleed the system without a "power bleeder". Unless you use a power bleeder you will never get a good pedal.....

[Charles Talbert]

The check valve answer sounds like what I have. As I stated prior , at the very last split second before the pedal comes to a rest at the top I see a rush back into the master cylinder and then I need to pump it up again.
It is a brand new cylinder from Vintage power wagon.
It was a bit disapointing to see that it was made in Taiwan.
I hate to pull a brand new item apart with out any clue as to how the check valve should be set or just where it is.

The wheel cylinder look like new ones. I peeled back the rubbers and peeked in. There was no seepage or rust,,,that I could see. They work smoothly and the brakes shoes are in good shape.
The odd thing is that if I adjust the rod that goes into the mstcylinder so that it has tension on it and keeps the piston from comming fully out , the brakes work just fine. It is only a matter of baybe 3/16 of an inch in from full out. I took the truck for a ride and the breaks work fine. The breaks release just fine , no draging.

I called VPW and asked about the problem and they say "there is no way you can bleed the system without a "power bleeder". Unless you use a power bleeder you will never get a good pedal.....

Let me say once again, be SURE that shoes are adjusted correctly FIRST. If you have any doubt at all of how to do it correctly, find out before you attempt to adjust; feel free to call or email me if I can help you, don't just assume they adjust like any other truck, they do not.

I have rebuilt many a brake system, and bleed without a power bleeder. The theory that a power bleeder is a must have item to get a good pedal is purely mythical. It simply is not true; the fact that they would make that statement really makes me wonder what is going on with them. A correct bleed procedure is absolutely necessary; and a good pedal is not a problem without a power bleeder. Sure it's a nice tool to have, but who wants to make that sort of an investment unless they are a frequent user like us for example.

Once you know the adjustment is RIGHT for sure; if the problem still exist, I expect the check valve issue is what you will find. The pedal adjustment you have done holding slight pressure on the piston is doing exactly what a proper functioning check valve would do. That is holding slight pressure in the system by not allowing the piston to fully return. It is not a good idea to continue to run it that way, you should repair it so it functions correctly if that proves to be the issue.

[Kaiser2boy]

How are you adjusting the brakes? Upper and lower adjustment?

I turned the heel adjuster while rotating the wheel until I felt resistance and then slowly backed it off until the wheel spun freely. I did that for both heel adjusters on each wheel and did the same with the toe adjusters.
It may not be perfect but it has to be very close.

I cant think that a few thousanths of an inch extra in brake shoe clearance would cause the pedal to go right to the floor.
If that were true it could happen at any time while you were driving as the brake shoes wore down.
I could see it if the shoe was 1/2 inch away from the drum but they are not .
The truck stops just fine and has no brake drag , no pulling left or right , no locked up wheels.

I will get to the botom of this problem sooner or later but for now it is a total pain. I just hate stuff that makes no sense.
One thing I did notice when I removed the old mstrcyl was it too had a "pre-load" on it. When I pulled out the clevis pin the plunger rod shot out about a half inch. ????

[Kevin Tighe]

Pumping brakes after each stop.

It sounds like a brake adjustment problem to me. I've got 3 - M37s now and spent way too much of last summer getting then all to stop. Be very careful to not make the push rod too long. That is just fooling you into thinking you've fixed it and you hav'nt. Sometimes adjustment is difficult because something in the drum is messed up. I bought a Mercedes Benz Unimog in September and more than 90% of the brake lining rivets were broken...that made it impossible to adjust the brakes properly. It almost has to be an adjustment problem. Good luck!