The Stovebolt Site Forums The Parking Lot Big Bolts Master Cylinder adjustment question

My '52 6400 has a completely rebuild or new braking system. New lines, rebuilt hydro vac, 6 new wheel cylinders, & new master cylinder. The brake shoes are adjusted just at a nominal drag.

Symptom: I would build up very good pressure by 3 brake pumps (roughly 1/2 a stroke). However, the brake pressure fully depletes the lines BACK INTO the master cylinder. I adjusted the brake clevis out roughly 3/8". The plugger (correct name?) is just behind the needle size hole in the master cylinder itself. Now the brakes won't disengage! haha. I ran out of time to reverse half the distance of the clevis so maybe that will fix it? With the built up pressure, the pedal doesn't even travel 1/4 distance.

Anyhow, looking forward to your thoughts for a proper master cylinder adjustment. Thanks!

The master should be adjusted so the piston JUST uncovers the bleed hole at the end of its release stroke. With the cover off the master, there should be a small fountain of fluid shooting up when the pedal is first touched. It should stop as the pedal continues forward almost instantly. Do not push the pedal hard or fast with the cover off, you will get a huge mess. Just touch the pedal lightly, with your hand if possible, while observing the fluid..

There needs to be a small amount of play where the push rod goes into the master cyl. piston. The piston needs to go back to the stop. If fluid runs back into the reservoir you either have air in the system or the shoes are not adjusted correctly.
George

You may have a master without residual pressure valve/valves in it. That is needed with drum brakes to prevent the rapid return of fluid from the wheels to the master when the pedal is released. Not needed nor desired with disc brakes.

Mac, how do I know if my master has or does not have the pressure valve set up you're describing? It was purchased new at NAPA.

The valves are pressed into the outlet port/ports, one or two depending on if the master has one or two ports and if it is set up for drums or discs. If they are there, you will not be able to see through the port with light behind or feel through with a fine wire. No forcing, if it is not there the wire would pass through as if it were a simple drilled hole. The valve will block a direct passage through. It looks just like a brass seat for the flared tubing end so that will be the same with or without the valve function. What master did you order? Is is a single port? If so it should not be for disc brakes. If it is a dual port then it could be all drum, disc/drum or all disc. All disc would not have valves in either port. I have never used them but supposedly you can buy external valves to add to the system. A brake re-builder should be able to install the valves in your master or maybe you could return it for exchange to NAPA depending on how "installed" it is.

I'll be back at the truck this Friday and I'll follow up with you Mac. Thanks for your wisdom.

If you used a master cyl. for that model truck, it will have a check valve built into the rear of it. That master cyl. was never used for disc brakes & a check valve is part of it. To check the function of the check valve, pump up the brakes & release the pedal. Wait about 30 seconds & crack a bleeder. A small amount of fluid should come out like it is under light pressure. Then it will stop coming out or just drip a little.
My opinion is that is not your problem.
George

I agree with George, before wasting time on the MC I'd start by adjust the shoes first...

Mike B

Thanks Mike but the shoes are adjusted as mentioned initially.

I'm back on the farm and am going to change out the driver cylinder as it was installed new ~2yrs ago. I attempted to rebuild all other cylinders at that time but had to buy all new as the walls were too pitted and leaked. The driver cylinder is not releasing pressure as it should where as the passenger side releases quick well and as expected. The driver side gave me trouble then too as it would lock up when I applied the brakes. Maybe there is something clogging that line too?

I have adjusted the brake piston clevis back. The results seem to be favorable but will know more shortly when the cylinder is installed on the driver side. I'll keep y'all posted.

Have you replaced the rubber brake hose at both front wheels (should do the rear as well)? They can look fine, but be swollen on the inside and act like a check valve.

Mike B

Yes. All three rubber brake lines have been replaced new from NAPA.

New question. Is it possible to have adjusted the shoes too tight pre-line bleeding? When I adjusted the shoes prior to bleeding all the lines, there was light drum contact and the wheels turned freely. I took the truck out for a 1.5 mile drive on the farm and the drums are tight. I actually had to open the rear bleed screws as the drums were locked tight. Something is wrong and way out of whack. It's can't be that dang hard to adjust these shoes, master cylinder, etc.

Isn’t there a special bleeding procedure for Hydro Vac?🛠

I thought that a pressure bleeder was needed.

However, it is not mentioned in this shop manual?

[It looks like a pressure bleeder was used from 1951 onwards?}

Thanks for the link. It's been a while since I installed the hydrovac and have forgotten the potential of a pressure bleeder screw atop the main unit.
I'm going to study this piece and check for the #1 bleeder when I back at the truck next. Hopefully I'll be back up later this week.

Test the Hydrovac...

On the first start of the day push the brake pedal down and hold it, then start the engine. As the engine builds vacuum you should feel the brake pedal fade to the floor slightly...if it does, it's working. If you test just after running the engine, pump the brake 6-8 times to bleed off any stored vacuum and then re-test.

If the drums are locking up and the lines are all new, then the issue is the fluid isn't returning to the MC. You might have the pedal adjustment to tight, not letting the piston return completely or you have trash in the MC blocking the tiny port.

If you suspect the Hydrovac it applying pressure and not releasing, disconnect the vacuum line at the Hydrovac and plug it so the engine runs correctly (now the Hydrovac won't work) and see if you still have the problem. It should now take a lot more leg pressure to stop the truck so be careful!

Mike B