ramcram

When I adopted my '86 ROW 5.0L S, it had many issues but one still troubles me.
On long distance country trips only, the brakes will come on and stop the car, with a VERY hard pedal. It feels like the motor is dying but when I pull off the road and back off the throttle, it would stop dead.
I suspected the ABS because the ABS light would come on [correctly] with ignition and go off with crank [correctly] but come on as soon as the car moved off. Previous owner had taken it back to Porsche a couple of times but problem still existed.
Anyway, my mate and I just removed all disks, calipers, speed sensors and the ABS pump. I had the calipers o/h with genuine kits, including new pistons. We had previously checked the speed sensors, so we replaced all of them with a known good set. Then reassembled it all with a different ABS pump, all new hoses, pads, anti rattle plates and new front disks, flushed the whole system out with metho, then new fluid.
The brakes now feel even better than before and the ABS system behaves as it should. My wife and I just did a 5,200km test drive across the country [Australia] and the brake drag issue is still there! Not as severe as before, which just makes it harder to detect. They come on after maybe an hour or so of driving at speed but now not enough to stop the car, just get very hot and stink. In fact idling in drive, it still creeps along ok but with a very hard pedal.
Pumping the pedal will not release the pressure until the ignition is turned off. With the ignition off, maybe three pumps on the brake pedal and it's fine, start the car and away it goes with no issues for usually the rest of the day.
It has never done it around town, so I had this idea that using the brakes regularly would prevent the problem, so every few miles, I would press the pedal enough to just make the pads touch the disks. Sometimes I could feel a tighter pedal which would go back to normal free play after a couple of applications but it still beat me on all but one day out of 6 country driving days.
When the brakes do come on, they can only be released after the ignition is turned off, so there is an electrical function involved, which would seem to rule out mechanical issues.
The only thing left that I can think of is the ABS computer?
I would realy like some ideas, or even better, a solution to this problem.
Thanks in antisipation.
Mike

WallyP

If the brake pedal push rod is maladjusted, or there is something faulty on the pedal, the outer cup in the master cylinder can be pushed in just enough to cover and block the compensation port (a small hole between the master cylinder bore and the fluid reservoir). This traps fluid in the system. When the fluid gets hot, it expands, slightly applying the brakes, which drag and generate more heat, which expands the fluid more, which causes more drag and heat, which...

No idea whether or not this is your problem, and no idea what turning the ignition off could do.

69gaugeman

If you unplug the abs (or remove the fuse) does it still do it? Removing one thing at a time is the best way to trouble shoot.

I once had a dodge neon that had the weirdest issue. When you stepped on the brakes they would stay engaged. Thought it was the caliper so put new calipers rotors and pads. It still did it. Replaced the master cylinder. Still there. It turned out the inner lining had detached in the brake hose and the flap acted as a one way valve. Replaced the hose and solved the problem....

Dannyfumi

Mike,

I had the same exact issue on my 87, I spent a ton of time and cash going after abs, bias valve, brake pedal, on and on. It turned out to be the Master Cylinder! Good luck

Dan

Mrmerlin

replace the master cylinder.
I dont know what METHO is but I dont think I would be flushing the system with it.
That said I would also suggest to use ATE super blue racing fluid. mixing different fluid can cause the seals to swell the MC will swell first mthus it may not be able to fully return to rest position thus what Wally said.
I have seen a few issues where the owner swapped in new fluid (usually a SYN type fluid)and flushed the system then about 4 months later the brakes were sticking on.
Swapping in a new MC cures the issues however the calipers should also have fresh seals prior to installing the new ATE fluid and bleeding.

Also if the bias valve has not been swapped then also swap this part as well,
they will get corroded and cause sticking issues

Dave928S

"Metho" is 'methylated spirits' which is ethyl alcohol ... known in the US as denatured alcohol.

Ramcram ... I agree with Mrmerlin re not flushing the system with metho. Even though it may not have any effect on seals ???, and I've used it for cleaning bores and pistons prior to assembly because it leaves no residue, it shouldn't be used in the system for flushing, and should only be used for cleaning of components when it will evaporate away.

It has a boiling point of 78 degrees C (below the boiling point of water), and any residue in your system will be having the same/worse effect as water. When your fluid in the system gets above 78 C, any metho in the system will become vapour, and be likely to cause the problems you're experiencing.

You need to flush the system to get rid of any traces of trapped alcohol, and you're going to have to do it more than once, using different methods ... and only using brake fluid.

Mrmerlin

I would flush the system with some fresh ATE Blue,
then replace the master cylinder,
then bleed it .

Also if the brake bias valve has not been replaced then it would be a good idea to swap it,
out as they do get corroded and cause the rear brakes to lock

ramcram

Wally, this is my dilemma. If the return port is blocked and the system is becoming pressurised, how does moving the pedal a couple of times disipated it?
If the system has become fully pressurised and won't release without the ignition being turned off, how does the ignition get to the master cylinder?

ramcram

Ah, I think you have a great idea here with the relay or fuse. It takes a very long test drive to check though.
I still have the prolem after replacing all hoses, so I think I can tick that one off.

ramcram

Dan, what was the link between master cylinder and ignition?
Mike

ramcram

'Metho' is metholated spirit, a non mineral based fluid used to clean brake components [and many other uses] during o/h because it does not attack rubber.
I'm using the same high grade brake fluid, from the same sealed drum as I used at the last 3 year flush.

ramcram

Dave, after reassembly and flushing with meths, we flush the new brake fluid through with a pressure bleeder [Blue Point I think it is, quality US unit]. Working from the furthest caliper from the m/c, the left rear on a rhd car, to the nearest, until a steady stream of fresh clean fluid has been expelled from each. This method in effect sees that the system nearer the m/c gets multiple flushing.
Boiling contaminant, below brake fluid b/p should make the pedal all soft and sqwishy and loose braking, wouldn't it?
My brakes go as hard as a rock, exactly as Wally is suggesting would happen by blocking the return port. As I replied to Wally though, at this stage I can only release the brakes after turning off the key, then pumping the pedal a few times and they are back to pefect againg . . . until next time, maybe the next day. The baffling thing is I can do a 1000km without it happening.
Dave, I just drove this car from Adelaide to the Gold Coast and back via Broken Hill, so you will have an idea of the roads/distances and only one day did it do it twice. One day we did 850km and another 940km, this longer day it didn't do it at all?
Baffling!
Mike

ramcram

You may be right on both counts as we haven't touched either of these units except to replace the reservoir to m/c seals, though neither of these units should be affected by the ignition.
I've got to go up bush [600km round trip] in 3 weeks and I'm going to firstly try the ABS dissable as suggested, then see where from there.

I'm very grateful for geat input from you guys.
Mike

ramcram

I forgot to say that the previous owner did get a new m/c fitted at the local Porsche centre not long before I got the car. That's not to say that it isn't the culprit now!

Leon Speed

It would be weird if the ABS system had anything todo with this. ABS releases brakes afterall, it doens't apply them.