stevesp98

Maybe - when I first stepped out of the car I thought I got a faint whiff of something but I didn't smell it again when I stuck my head under the hood and then got down underneath the car - I used the parking brake to stop so the whiff I got could have been from that.

I used the brake pedal while I was trying to stop but I just stayed down with no feeling or resistance. I didn't touch the pedal again until I was sitting for about 30 minutes and then I put my foot on the pedal without thinking and it actually kind of surprised me when I popped right back up.

Speedtoys

No..pedal came up, the nearly fully failed seal is sealing in the M/C bore again.

A hot wheel wont collapse the M/C. Two hot wheels wont, four hot wheels wont.

dr bob

OK, how about this:

There's a return spring on the pedal that should pull the pedal back regardless of what happens forward of it at the booster or the master cylinder. The spring is 928 423 531 03, a coil spring that hook onto the pedal near the pivot pin. Make sure it's there.

The master cylinder depends on an open port to/from the reservoir when the pedal is at the full-up position. At that time the pistons in the master cylinder are fully retracted so the fluid from the reservoir can flow into the bores in front of each system's cup seal. Move the pedal down slightly and that port closes, and pressure from the piston moving is now all forced to the calipers. At the top position, the pedal pushrod needs to be clear of the master cylinder piston by a tiny bit. You should be able to feel some slight free pedal movement before the resistance from the master cylinder piston is felt. Do the check with engine off, and half a dozen pedal-presses so there is no assistance from the booster.

GregBBRD

Tandem master cylinder. 1967 was the last year of single piston master cylinders. The odds of both the front and rear piston seals failing at the same time are very low.

However, if the fluid boils, the pedal will go to the floor because of how the returning pressure affects the piston seals on the plungers.

These cars have some very strange failure modes of booster and master. Some booster failures will actually pull down the pedal and cause the brakes to be applied. The brake fluid gets very hot, very fast.

Also common on S4's are the front caliper pistons sticking....especially common when the pads get thin. The brake fluid gets very hot, very fast.

These cars are also now old enough to have "hardening of the brake hoses", where the interior rubber of the brake hose swells so much that fluid will not return....keeping the brakes applied. Same high heat situation.

This car needs some careful testing to determine the cause of this failure.

Of course, a new booster and a new master is never a waste on a 30 year old car....

stevesp98

On further review, none of the calipers seem to be sticking - at least not now. However the front pads are pretty thin so out of an abundance of caution I'm going to go with a 3 pronged strategy of replacing the M/C, flushing the brake fluid and replacing the front pads. Then I'll start testing things and see where we're at.

alex70

I had a low speed failure when in Anaheim. I should have heeded the car's warning when it seemed to pull to one side slightly.

Next day pedal to the floor, escape was via clear left turn lane and green light, e-brake to stop. I think it was Devek that replaced MC - not a good feeling, especially on how hard I would push on the off ramps.

Definitely go through the whole brake system to find the culprit.

Good luck!

dr bob

Add new brake hoses to your replacement list now, while you'll have things apart and you plan a full system flush anyway. There are more than a few threads here on some of the watch-outs and the procedure. The old hose get tired as Greg points out, and start to come apart internally. Theycan become little ballons, and the internal pieces turn into unintentional check valves. The better replacement lines will take care of that, or go with new stock pieces.

James Bailey

And if the booster is at fault you get to do the master cylinder and bleed a second time. Or if a faulty brake hose.
Which is why Dr Bob suggests trying to determine the actual cause of the problem.
I think it is highly unlikely to be the master cylinder but that was based on my own personal experience and the feedback from several customers.
My son in law changed out the booster and also the master because when you have a 20-30 year old master cylinder off the car it is silly to put it back on.....
This is also a good opportunity to renew the brake hoses.

Imo000

Stick with stock flex lines. They are DOT proved (unlike many braided lines). If you rebuild the entire system (calipers, MC, and booster) then there will be not much left that can fail alter on. You should be able to tell the pad that was stuck just by looking at it when you pull them.

polecat702

This happened to me on the Vegas Strip. Replaced the MC, and cured the problem. I've got a new spare in my box of spares just in case.

Got a pulsation now, so it's new rotors in the near future too.

Charley B

Classic fluid boil.

Randy V

Auto or manual trans?

DeWolf

I have had this happen to me. Not in a Porsche but in an Australian Ford Falcon. It was an EF model. Came up to intersection and no brakes whatsoever. Went sailing straight through a red light. Got the car stopped and changed my underpants. Fifteen ,minutes later start the car and brakes fine. Take it to mechanics. He drives it and comes back ashen faced and yells "that f%^&ing thing almost killed me". Turned out to be the abs computer on it's way out.

stevesp98

Auto

stevesp98

Done, out of an abundance of caution I'm essentially replacing everything and then we'll see where we're at.