drwhosc

Ok I posted about my idle woes, but prior to getting that worked out, On my last drive, my brake pedal went to the floor, and I lost my brakes. Luckily it was only a coupe of blocks from home so I did not crash into something, but I can see how scary things can be with no brakes.

So I am looking for advice. How do I troubleshoot this. My basic instinct says replace the master cylinder since the pedal goes to the floor and I have no pressure.

I looked at the fluid res, and it has fluid in it. I do not see any leaks and I can not get pressure when I pump the brakes

I did have a bit of drag prior to the loss of brakes, and I am not sure if that could have contributed. Possible fluid boil ??? the brake system was working well just prior to the failure I wen to apply the brakes, and then the pedal just went to the floor with little to no effect. It was a sudden failure.

Again, I do want to thank folks for their help, I find the knowledge on this board invaluable for the hobbyist such as myself.

drwhosc

Well guys I thought just for yucks let me go out an push on the brakes, and I have pressure again. So prob not the master cylinder and more likely a sticky caliper piston with a boil. Any way I can trouble shoo this type of issue...

Speedtoys

If you were not doing a lot of super hard braking..you didnt boil.

You need a new master cylinder..you may also have brake fluid under the carpet on the drivers side too, or up on the firewall under it.

drwhosc

will check for fluid.... thanks and no not very hard braking at all. i was luck yo be in a situation where i did not have to brake hard, and don't want to go back there

linderpat

first let's fully diagnose before buying parts, especially expensive ones like a master cylinder. I assume this is for your 88 S4. Since you said the fluid level appears to hold steady, then you can rule out a breach in the lines somewhere (a line leak and fluid loss would cause the problem you are describing). Nevertheless, double check this.
When is the last time the fluid was flushed? Have you done anything else with the brakes, like change pads, etc? Also, when you drive it, does one or more of the hubs get real hot (indicating a stuck caliper)?
Do these things and then narrow the issue. It may indeed be a bad MC, but try to make sure first.

drwhosc

Thanks Linderpat, I agree on not buying parts I don't need, and that is why I am looking for advice. I plan to jack the car up and get it back on stands. Nothing done to the brakes, but did do the front suspension. I suspended the calipers intact when I had the singles off, so I did not go into the brake system

When I put it all back together, and went for an alignment, we noticed the passenger front wheel did not turn easily. it would turn though, but not freely like it should. I have to further diagnose this issue. I do not have a temp gun and I am very hesitant to drive not knowing if my brakes might go.

I have never flushed the system, and I am thinking that may be the next step. I will also see if I can turn my huns freely, and if not, maybe bleed the brakes, but push the pads back in a bit.

Kind of busy this week, but hope to get to this weds evening or thursday and at least take a look

linderpat

Well those are a couple of good clues. First, flush the system. https://rennlist.com/forums/928/4691...re-w-pics.html.
Brake fluid is hydroscopic - it absorbs water, which boils at a much lower temp than the brake fluid, and when that happens, there is no stopping at all - pedal becomes mush or just drops to the floor. Old fluid if heated sufficiently will do this. If the front caliper is sticking, that could generate sufficient heat even without hard driving to boil up the old fluid and cause your problem. Do a flush and really test that caliper. Note, the calipers can be rebuilt easily and cheaply with the proper kit from one of our good 928 vendors. There is a very good DIY by Dwayne (see Dwayne's list for the topics and the correct DIY)

123quattro

One boiled caliper will not cause a complete system loss. Just 1/2 circuits at worst. Any time the pedal goes to the floor with no fluid loss I expect a failed master. However, being this is an ABS car, if you have loss of wheel speed sensor output you can get locked out from the brakes. That gives a hard pedal at about half travel. Are you sure the pedal went to the floor?

drwhosc

Oh yes sure the pedal went to floor. I lifted the car today, and spun the hubs. Both fronts were sticky. I pushed the pads in a bit, and had free spin. I applied the brakes, and things were tight again. I do think I have sticky pistons, and will do a camper rebuild. Does not look too bad.

I also cracked the bled nuts today to drain the system, and fluid was pretty dark, so I wonder if it was ever serviced. I am thinking about just replacing the master since I am going to have the brake system apart.

Speedtoys

Ya..the pedal to the floor can only mean a -very- few things.

If everything is dry at the wheels, it fell because a master seal is crying for attention.

SMTCapeCod

I dunno...I had my brake fluid exchanged last winter. a few months ago, had the pedal go to the floor. It did not pump back up well initally, as one might expect with air in the system. However, the pedal gradually recovered. I replaced the master, found a minor seep at ABS block and so replaced the nearby proportioning valve...and later had another instance of the pedal going just as soft. Seemed to happen same time as the ac went warm...but loss of vacuum should cause a firm pedal.

Speedtoys

With a system that's been fine for...ever, it would take a lot of random air getting in there to do this once, and then not do it again..because it magically dissapeared.

Air in the system would be a static issue, repeatable..not random.

SMTCapeCod

Well conferring with the ABS Information Technik isn't much help, basically a walk through of testing with the dedicated ABS tester. May check the relay on the pump itself.

drwhosc

How does one check the abs pump. I am wondering if I have increased baseline pressure causing my pads to stick. my pistons are movable, so they are not stuck.

Jim Devine

The brakes are a sealed system. Dirt doesn't get into the fluid. Dark fluid is usually from rubber seals in the system deteriorating. How dark is the fluid in the M cyl reservoir & any black stuff on the bottom of it?