The Stovebolt Site Forums The Parking Lot Big Bolts Calling All Brake Experts...What am I doing wrong?

Background: 1968 C60 full-length schoolbus. Sat for 4-5 years until I bought it, and has no brake pressure. Have been trying to bleed the system for a few months, without success.

Brakes: 4-wheel drums with two separate lines/bleeder valves on each wheel, one top, one bottom. Split master cylinder. The inside of the master cylinder cap has two rubber boot looking things, one of which says "fluid side". There are two large cylindrical tanks underneath the driver's side of the bus, toward the front - are these vaccuum boosters?

Symptoms: little or no brake pedal. in the process of trying to flush the system, have had sporadic pedal improvement followed by complete loss of all pressure again. sometimes when I go to refill the master cylinder, the fluid is foamy from air bubbles.

Method: per the wisdom of various stovebolt posts, I have been working from the wheel most distant from the master cylinder to the closer ones.

-I fill both sides of the master cylinder about 2/3 full, and retighten the cap.
-open the top valve on the right rear wheel
-have a friend pump the pedal to the floor 4-5 times and hold it
-repeat the previous step several times, usually a lot of air and some fluid comes out
-on the final pump, the friend holds the brake pedal all the way down and I close the valve
-repeat steps on bottom valve, and switch back to top valve several times.
-work my way around the bus (sometimes only the rear two wheels in a given setting), but sometimes I switch wheels while there still seems to be in air in the lines of the current wheel.


I dont get it, it seems like whenever i make progress, soon afterwards the pedal gets even worse. Should I be pumping the brakes with the bleeders closed before I open them? Is one side of the master cylinder supposed to be dry?

PLEASE help me, I need super-detailed instructions on how to properly bleed these brakes and any tips on what i might be doing wrong

You need a new master cylinder. It's inner seal is bad and is sucking air. Even if you get it bled, it will soon have air again. Put a rebuilt master on if you can find one or have yours rebuilt if you can't.
Good luck,
Scott

When you bleed brakes, don't pump the pedal numerous times with the bleeder open. What I do is I place my finger over the open bleeder and have my helper slowly pump thepedal. As soon as fluid appears, I call out and they stop and I close the bleed. This gets the job done about 95% of the way.

On your method of bleeding... once you have fluid coming out of the wheel cylinder you have to close off the bleeder valve...have your friend pump to build-up pressure...crack the bleeder until the friends foot is ALMOST to the floor.. close the valve quickly...repeat until no bubbles come out. A one way bleeder valve really helps. When do you stop?... When the pedal has 'feel' to it. You may have to work your way around the truck a time or two before you get what feels like a firm feeling pedal that stops well before the floor.
**don't forget to keep checking the m/c level. Nothing worse than running out of fluid and having to start over.

After all this work and not getting any where, are you sure the lines are all good? They love to rust from the inside out! You might have a small pin hole letting air in. If your bus has a Hydrovac unit it also has bleeders. It might be time to get a Shop Manual because the order you bleed is important.

Good Luck!

Mike

First, what you mean by "fluid side" on MC? Do the big trucks in 68 have dual chamber Masters like small ones? Isn't there a Hydro-vac maybe? This is another master and a vacumm canister to assist the system, making them Power Brakes. If you have a dual system, then maybe the Proportioning valve is to one side, blocking the other and needs to be re-centered? This is done with a small button at end of valve. Valve has four lines to it and a wire to turn on the dash Brake light. usually mounted near master or from cross-member on front? Maybe you should get big truck manual, school bus might have odd safer type system and require diff procedures??

School buses of that period had the same braking system as a truck of the same model number,single master cylinder,single or dual Hydorvac boosters and two brakes cylinders per wheel.I'm assuming the bus specs would be the same pretty much in any eastern state as here in NY.I believe medium trucks had a single master into the early 70's
I always used a power bleeder,didn't bleed the Hydrovacs separately unless there was a problem after the brakes were bleed as mentioned.Trying to hand bleed brakes on a long bus is very difficult at best.
Of course,the Hydrovacs or master cylinder may have a problem.

Instead of pumping the brake try this..open the bleeder on top of the booster leave the lid off the master cylinder. wait a few minutes and fluid should start coming out of the bleeder. make sure you keep the master cylinder full. when the fluid starts coming out, close the bleeder. then move to the other booster..repeat.
When you bleed the wheels instead of pumping the pedal, open the bleeder, push the pedal down to the floor, close the bleeder, let off the brake pedal. repeat on each wheel until you get a good stream of fluid. pumping is just causing more air bubbles.

You need a new master cylinder.

thanks for all the great suggestions. does anyone know where i can get a shop book for a truck this old?

PS - same question on the rebuilt master cylinder...what kind of places would carry such an item? truck repair shop? junkyard? any online resources?

Carquest has a good line of reman master cyls. They get a delivery from the warehouse every day. I have no connection with them but have used them for many years. Great service. Be shure to bench bleed the cyl. before installing it. Don g,s sugestion on gravity bleeding is very good. Just never let the MC run out of fluid. You have 2 boosters that must be bled also.

If the master cylinder was bad...how would he be getting pressuse when he bleeds the brake?
I will bet that a new master won't fix it.

But he doesn't have "good" pedal.

True anew master cyl won,t fix it but he never will with an old one. It could be drawing air in on the upstroke & the rubber parts are old & detereated.

I have had a partial pedal at times, but right now I have nearly none...I'll see if I can build up enough pressure with the old master cylinder to at least clear the lines temporarily, and see how long they hold, to determine wether I need a new master cylinder or not...I'm sure it's not an original part, as the bus was run up until about 5 years ago