Brake Booster Issue

[poundman]

Question. I have issue with my recently purchased 1972 bus that has been setting for over 20 years in storage. Engine not running yet but the brake pedal is hard as a rock and will not budge when I try to press. How can I troubleshoot the issue with no working engine. I am not sure if the problem is with MC or the booster. PO said it needs a new MC but I am not sure if that is the case.

What is the latest opinion on replacement vs restore?

Thanks.

[W1K1]

[aeromech]

If a visual inspection fails to find a cause I would probably remove the master cylinder. Press on the pedal. See what happens. In my mind, the brake system is going to need a full rebuild anyhow, so bleeding is going to be necessary anyway at the least. I’m thinking that you have rust inside the mc causing your jammed pedal. Water gets into the brake fluid and after a long time would freeze the spool in the mc
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[poundman]

Thanks, I suspect the MC is the issue but I will remove and test the pedal. I think I will have them both rebuilt. I have sent email to Karps. Any other suggestion for rebuild. I am in Houston area but don't mind shipping.

[aeromech]

In all my years I’ve never seen a frozen brake pedal mechanism and I think other than a visual inspection and a quick squirt with some spray lubricant, you’d be wasting your time disassembling your pedal.

If you want, you could remove the long rod from the booster to the pedal and then try your brake but it’s even easier than that. One end of that rod has a fork and a pin. Remove the cotter key, remove the pin, then pull down on the rod and it will disconnect to your pedal. At that point you’ve separated the system in half.

The rod is 25A in the illustration
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[poundman]

I removed my MC and disconnected the pedal linkage. My MC and booster are definitely corroded. The booster was full of brake fluid. I have another question. Where does the vacuum line from the back side of the booster attach? And where does the air hose from the front side attach? thanks

[aeromech]

The front side is just a vent to atmosphere. The rear goes to your engine. What year bus do you have? You might add that to your signature line along with other info maybe, like location
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[poundman]

[aeromech]

I think you’re missing my point. Go back to your preferences and fill out your location and year of your bus. Then, when you post people will know that info and can better help you.
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[poundman]

[orwell84]

Just adding my own experience to this thread. Brakes were feeling a bit weak especially when coming to a complete stop, along with an increase in idle speed when brakes were applied. Went through all the vacuum hoses and connections to rule out a simple leak. But fairly clear it was the booster.

Brazil booster was the best I could find at the time. I was aware of the fitment issues from this thread. The studs are in a different place compared to the original. It’s not just a matter of drilling new holes. The original bracket is too narrow. I bought a strip of 1/2x1/8 steel from the hardware store because I was too lazy to cut metal and welded them on to the original bracket. Made a pizza box template with old/new holes and drilled new holes. Took me more time than expected but it’s a solid repair.

I had a used booster I got from the classifieds but it was too mauled for a rebuild.

Sometimes you get lucky with the classifieds. Sometimes you get burned. Was advertised as booster from working bus. Word of advice when parting out buses: Pull parts before it goes to the crusher, not after. And don’t use a splitting maul to remove parts.

My original booster looks like a good candidate for a refurb. It’s all there, just leaky.

The Brazilian booster works great. Stops on a dime with no pedal effort. This is the way servo assisted bus brakes should feel. If you have been driving for a long time with a leaky booster like I have, your first stop at speed might feel like you’re gonna get thrown through the windshield.