Funky brake issue

aft911 · 05-25-2008, 06:54 PM

I have a 74 Carrera, exclusively a track toy. It sat idle for 6 months before a DE I did in May. So I was not surprised when I had some brake issues -- a good bleeding took care of that and the brakes were fine for the rest of the event. I did not do anything to the braking system after that event, which was only 3 weeks ago.

But the problem reappeared today. Same type of symptoms -- everything is fine for several laps and then next thing I know I'm pressing nearly to the floor and getting almost nothing, but it's fairly inconsistent. So it seems like air got in the lines again?

But after only 3 weeks? Sound like something more systematic?

Thanks in advance for ideas to explore.

Flieger · Flieger's Garage

Check to make sure the caliper flex lines are not deteriorating from the inside out. It could also be a master cylinder piston seal.

rusnak · 05-25-2008, 09:20 PM

need more info. can you get pressure by repeatedly pressing the brake pedal? Or does the pressure build, and then pedal slowly goes back to the floor?

David in VA · 05-26-2008, 09:26 AM

I had a similar issue after a track weekend which bleeding did not help. Turned out to be a bad master cylinder. Had it replaced and spent the last weekend on the track with no issues.

aft911 · 05-26-2008, 12:25 PM

Quote:

Originally Posted by rusnak

need more info. can you get pressure by repeatedly pressing the brake pedal? Or does the pressure build, and then pedal slowly goes back to the floor?

Thanks to all who have replied.

Rusnak -- the pattern is that everything's fine for a few laps, then suddenly I have no pressure until I'm near the floor -- doesn't completely disappear but 80% of my brake travel has no pressure. Pumping (repeated pressing) doesn't seem to help a lot. Once it starts this problem, on a few corners the brakes will be fine and then practically nothing... even at very low speed like coming into the pits.

Flieger · Flieger's Garage

I think the master cylinder is worn out. If the seal between the piston and cylinder bore becomes worn, the piston will not push fluid well until it reaches the farthest part of the bore which is not normally used and suffers less wear. Possibly your seal has a crack/cut in it that matches up to a score in the cylinder bore only at certain times.

rusnak · 05-26-2008, 01:22 PM

it certainly seems like a master cylinder internal leakage problem. but wouldn't this problem appear when the brakes are cold too?

Flieger · Flieger's Garage

I thought that, too, but something says "bad master cylinder"

Maybe as the brake fluid and pads heats up and he needs more pedal travel and force, the master cylinder seals cannot take the pressure and give up in that worn portion of the pedal travel.

What if his flex lines are swelling, allowing more fluid to be pumped into them over a few laps without much loss of pressure, until the fluid tank gets low enough that the master cylinder starts pumping air. The level may only be low enough that it does this in turns where cornering forces force fluid away from the outlet to the master cylinder.

I am just thinking of anything that may help.