Braking diagnosis

[clint78z]

The old archives at ZCAR.com saves my day again, I was having the worst time trying to get my toyota 4 piston calipers, and 15/16" master cylinder going. The pedal would drop half way down then instanly lock the brakes with dead solid pedal pressure. It appears a little piece inside the booster can fall out when adjusting the little rod on the end. New booster is on it's way, little bugger I always seem to find the painful stuff that never happens to anyone. I smacked it still didn't work

 

Here's the diagnosis

 

The problem you describe is indicative of a what I call the "crash pad" falling out from under the plunger which protrudes out from the front of the booster. You must be very careful not to pull on this plunger as the crach pad will literally fall out into the interior of the booster. This pad is simply a rubber pad which is appox. 3/16 inches thick. It acts a cushion between the diaphram vacuum plunger which is attached to the brake pedal and the external plunger. You should always double check the clearance between the external plunger and the inside of the master cylinder piston. There should be no more then .030 inch clearance at rest between the two. The loss of the crash pad causes this measurement to increase to unacceptable levels. By the time the plunger has finally contacted the piston inside the master cyliner, the booster is applying panic braking levels of vacuum assist to the brake system. Ergo the sudden lock up of the wheels.

It is not possible to repair easily, Get another booster and be careful with the plunger.

[clint78z]

Braking is resolved, diagnosis was correct. Talk about something to make you scratch your head. The little reaction disc (rubber bumper) fell out inside the old booster. I got a rebuilt one for $160 cdn, no vacuum leaks on this one. The brakes have been restored tadda !!