mayday1

I donīt know how long the old brake fluid was in there since I own the car only for two years, but the drained fluid looked very much like the new fluid that I put in, there was no sediment or dark colour
coming out when I bleed the brakes yesterday. I used up two litres of fluid, so I assume the system was thoroughly flushed yesterday. I worry that the ABS may have a problem though, since that would
be expensive and beyond my abiliites to fix. But I know the ABS
itself worked since I autox it every couple of weeks and triggered the ABS frequently, I marvelled how well it worked since it allowed me to brake really late nd still turn the car at the same time. If the actual ABS
functions well, should I still supsect it ? Or can I just try simpler things like rebuilding the front calipers? What I donīt understand is why the problem only shows up when the brakes
are hot, but works fine when the brakes are not hot.

Mike Murphy

fpb111

If your brake pedal is not fully returning*, the pads won't retract, the brakes will drag more as the fluid heats up and expands with no place to go (master cylinder blocking the feed hole from reservoir). Make sure that your brake pedal is returning to the stop. If the pedal is totally free then I think you may need to replace the master.

*This can be caused by water/debris contamination in the vacuum assist.as Ahsai mentioned more prevalent in 1999.

mayday1

I wasnīt able to find the TSB mentioned by Ahsai about the water contamination problem. Can you briefly explain what it is? My car at least in the past two years have not seen water other than the very occasional car wash, and it is a california
car all its life, but there may still be water problems.

I took the front calipers out tonight, it does look like they were worked on a few times in their life, all the nuts for the hydraulic tubes are slightly rounded, and the clear-coat is flaking off. When disconnected, I can easily push all the pistons all the way in.
I havenīt pulled out the pistons to inspect yet, it is too late at night to fire up the air compressor. Iĺl pop the pistons tomorrow.


One strange thing is that when I disconnected the brake hoses in the front, I expected brake fluid to gush out. Instead there was nothing coming out, and only a few drops dripped on the ground after a few minutes. Is that a concern?

808Bill

Does your brake peddle return all the way back up when you let off?

dgmark

When the brakes are hot and the fronts are binding, crack the lines at the master and see if this releases the pressure on the fronts, if it does the problems in the master, if not repeat the same and crack the bleaders if the pressure releases the problems in the hoses, if not the pistons are binding.

fpb111

SpartaEvolution

Though the brake lines degrade over time and may clog or have buildup within the line to cause back pressure etc, because both the fronts are equally affected, it seems that the issue may be higher up in the system. The brake booster may be getting stuck or may be leaking that is leaving residual pressure as the system gets hotter which would explain a majority the current situation.

mayday1

I managed to pop off three of the four pistons off one of the calipers with an air compressor. I couldnīt get the 4th one out. The three that are out look
pristine, even the outer dust seals look perfect after cleaning off the brake dust. I havenīt pulled out the inner square profile rubber rings yet, but the
bores and the rings look perfectly fine. Donīt know if the 4th piston is also perfect, or is the one causing problems.

I donīt know if I should bother rebuilding them since the calipers may be better than my rebuild job.

Unfortunately I didnīt check if the brake pedal returns fully before I pulled out the calipers, so I canīt check until the calipers are back in.

So far the only insights I have is that the rear brakes seem to be fine.

Porschetech3

Ofcourse you should "rebuild" them. If the pistons and bores are perfect then the rebuild will only be cleaning and replaceing the o-rings and dust boots. The square o-rings can harden with excess heat and time, the o-ring actually flexes under pressure and when it relaxes it with draws the piston in a small amount to lessen drag, so if your old ones are hardened the new ones will help.

mayday1

Youīve convinced me to rebuild, new softer rubber o-ring may help the pistons to retract.

But now Iīm suspecing the problem to be upstream.

BTW, I popped off the 4th piston, it too is pristine except for the scratches I made when trying to pry it off with screw drivers :-(

mayday1

A quick update:

I rebuilt both front calipers by replacing all the piston seals and dust boots. There is a bit of improvement. When cold the car acts normal with no drag. When fully warmed up there is a slight
drag as noticeable when you reach an almost full stop, the car does a full stop by itself. I just did a 8 mile drive of mostly city, speeds from 20mph to 50mph. Afterwards the front rotor temps
were 100C, the front hubs at 110C, the brake calipers at about 60C. The rear rotors were about 55C, the rear hubs also around 50C so the parking brakes were not dragging, and ambient temp was 20C.

A summary of what were replaced: all four rotors, all brake pads, borh front brake hoses, and rebuilt both front brake calipers.
I guess I have to check out the master cylinder next. The brake pedal seems to return to rest position after braking but I canīt tell for sure with my foot, and I canīt see the pedal when Iīm braking. Will get a friend to
look at the brake pedal while I brake to see if it returns to position.


I

Ahsai

When you feel dragging just use your right foot to lift up the brake pedal and see if it makes a difference. If the dragging goes away, it's likely the brake booster.

mayday1

Iīll do that next time I drive duncan.

I removed the plastic panel covering the master cylinder. It looks like the master cylinder has two lines coming out. One goes straight to the ABS unit, the other goes through a 2" long black metal cylinder before
going also into the ABS unit. The ABS unit has two lines coming out into the driver side fender well. Can I assume the line from master cylinder directly to ABS is the front brake circuit, the other line that
goes through the black cylinder is the rear circuit and the black cylinder is some sort of pressure regulator thingy? The two lines coming out either go to each diagonal brakes, or one goes to the front brakes,
one goes to the rear brakes. Either way I donīt see how it can prevent a wheel from locking, Shouldnīt there be 4 lines coming out of the ABS to each individual brake so that each brake can be unlocked
individually in a skid of when hitting an ice patch?

I need a lesson on our ABS.

mayday1

Loosening the brake reservoir cap seems to alleviate the brake drag, but I donīt know if it was just placebo. Can the cap be faulty? I know it is vented but looking at it I donīt see how it vents hence my
experiment with loosening the cap to improve venting.

This afternoon I jacked up one side of the car in my driveway with the engine running after a few miles of driving, and hand rotated both the front and the rear wheels without too much resistance. Then I stepped on and released the brake,
and there was a lot more resistance when I hand rotated the wheels afterward. I lifted the brake pedal by a couple of mm, and the wheels were much easier to rotate.

So basically the brake pedal returns to the stop, but all four brakes are dragging, If I force the pedal up 1/8 of an inch or so, and the brake drag mostly disappears until the next time I brake.

From reading various posts, it seems like the vacuum booster is the likely suspect. There is no signs of water damage to the booster, the two long T45 bolts holding down the booster are nice and
shinny without any sign of corrosion. There is no sign of water damage near the booster/pedal/horn area (it is a california car).

Can the master cylinder be the problem? Replacing the master cylinder isnīt difficult nor expensive, but removing the booster is a lot trickier since it may not come out without first removing the ABS unit.

Whether the problem is in the master cylinder or in the vacuum booster, why does the problem not manifest itself when the brakes are cold? It isnīt that the MC or booster get heated by the fluid inside the calipers.

Can the problem be as simple as the adjustment between the booster and the pedal being off? Perhaps even with the pedal at rest, the master cylinder is still being pushed in a very tiny amount to cause a slight drag?

Should I go ahead and rebuild the rear calipers just in case :-)?