1968 vette 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

 

New rubber lines?

Check the M/C booster connection. make sure the rod to the M/C is adjusted so the bleed port is opened.

 

Good old corvette brake problems.

Any brake fluid on the ground or calipers?

If no fluid is leaking OUT, then air is probably leaking IN. Check for leaks at all of the rubber to steel interconnections. Also make sure your copper washers are all sufficiently compressed where the flex lines connect to the calipers.

I gave up using vacuum pumps, as I found they can cause leaks in the caliper pistons. If you used a vacuum pump, you might want to pull your calipers one at a time to verify they are not leaking through the lip seal.

I use the old gravity method, followed by getting my wife to pump and hold (the brake pedal), and bleeding this way.

Also, it wasn't clear to me if you changed the flex lines, or the hard lines. It's possible that you might have a pinhole in one of the hard lines. A major PITA to change. If you changed the hard lines, you're probably OK.

 

this sounds like a multi-symptom problem. you state that it works fine after pumping it up. if that is the case then it cant have much leakage. if you hold the pedal does it creep down? any loss of fluid seen? how did the old fluid look? do you have a brake light on?
you may have a bit of trash in the prop valve and may need to re-bleed and flush the entire system. you may also have some pad travel issues. if the pad travels freely then they naturally find a sweet spot and give you firm pedal. if they stick then they drag and loosen to give free space that is taken up after you pump.

a loose wheel bearing will give you all the symptoms you mentioned. jack the car up and grab the wheel from the top and bottom and pull/pussh to find movement. movement will cause the pads to push back and create the symptoms you explained.

 

Yes, i changed all the lines, rubber and the steel ones.

 

Yes, the old corvette brake issues... No fluid on ground or the new O ring seal type S/S calipers... Checked and re checked all connections...I also bleed about 2 quarts via gravity and pressure bleed ( person in car) method. Most are still saying not a PBB boster / vacuun issue and a M/C issue even though that test fine?

 

Bleed / flsuhed lines muliple times... Did the wheel bearing check, no movement.

 

Based on your described symptoms, I would think the problem would have to lie somewhere in the master cylinder. If the rest of the system is sealed, the only place where force on the pedal might be lost is in the M/C. There could be a problem with an internal seal, or a machining error in the M/C valve bore or on the valve, itself. If this [exact] problem didn't exist until you rebuilt or replace the M/C, I would suggest changing it [again?].

 

Unfortunately… This was the (exact) problem i had when i started to replace the entire brake system... At this point, it would appear to be a mystery, or i have simply replaced w/ more defective parts, which just seems to be hard to believe.... I try not to be just a parts replacer... But I just can't seem to solve this one, no matter what i read, ask and have already tried? I find w/ these type of problems its usallly something really small, but single point failure never the less.

 

After 2 brand new mastercylinders,o ringed calibers,steel braidedines,new pbb, and a new proportioning valve.I had to pull the mastercylinder twice and bench bleed it tilting every which way you can.I have a decent pedal.Not grate but decent.They can be a great pain the butt.I'm still going to bleed it some more.People can't say I'm not a patient man.