1968 (427-3x2) brake issue:

Had a brake pedal problem as follows.... When car sits for a few hours brake pedal goes to floor... I can pump up the brake pedal, then the pedal feels fine and stops fine ( perhaps a little grabby~).

So I replaced with New: M/C, PBB, calipers, brake lines and converted to Dot 5.... Bleed system (gravity and pump method... car seemed to be fine then reverted to the same problem.
Only other item items worth noting: I can hear a hiss (air sucking noise isolated to vacuum canister –not sure if that is normal?) located under the M/C driver side, but believe this is only the reserve for the headlights, wiper door while engine is off or while driving at low rpm’s...I also tested the PBB, (pulled off the hose/fitting- air was sucked in ( check value works fine) and M/C ( blocked off hose fittings holds pressure no problem there... I also feels like the brakes are dragging a little (won't roll back on incline, but seems to release fully after it sits.
I had spoken to more people and read more blogs then I care to mention….Starting to run out of ideas. Thoughts on problems or diagnostics I can run to narrow down?
Some of my last idea’s
• Vacuum canister issue?
• Proportioning valve issue?
• PPB Issue (internally)? - I really would like to be certain if this is a PPB issue a project to change again


Any insight or thoughts greatly appreciated!
Thank you

 

How long after you changed everything did the problem come back? Did you switch to calipers with standard piston seals or O ring seals. If you have one or more rotors with excessive run out they may cause air to be sucked in at the pistons.

 

Anything teed off the vacuum line from the engine to the booster? There should not be.

 

The problem came back after a driving few miles....I changed all 4 calipers (w/ the O ring style piston seals) and new brake pads & rotors too.

 

Nothing teed off the PPB, the vacuum line to PPB check valve connected directly to engine manifold..However, the connection to the manifold is a double tee one for the PPB the other going to the vacuum for all the other items ( vaccum canister, headlights, etc...)

 

If you bleed the brakes do you get air out? Generally, if the pedal goes to the floor the cause is either air in the system or excessive rollback/knockback of the caliper pistons. Do you have to drive it to get the pedal issue or just sitting?

Gary

 

I have bleed several times ( gravity and pressure) and would appear no air comes out just a steady stream of dot 5...when the pedal does go to floor after sitting, i can pump back up while sitting.

 

Check the booster push rod length. If it's to long, the MC pistons will not come back to the 'off stop' and lots of strange things can happen (brakes drag etc.) Loosen the 2 nuts holding the MC to the booster and 'feel' if the push rod contacts the piston by gently pushing it back against the Booster. It should not touch.

Gary

 

I have the same issue with my manual brakes. I havent figured out the problem yet. I'll try to let you know when I come up with a solution.