michael_j_brown

Brake experts, here's your chance to help me (and others) diagnose significant brake issues. I'm a long-time Porsche owner with a 928s, 914 and 911, this being my first Boxster.

Car:

• '99 Boxster
• Purchased 3 weeks ago with 50k miles
• Always garaged and in near mint condition

The car drove perfectly for the first week of ownership then....

Original Issue:
Driving at 45 mph car developed major drag, causing the car to almost stall. I immediately thought the clutch had gone out but depressed the brakes and the drag stopped. Over the next few days, the problem persisted and got worse.

Attempted Resolution No. 1:
Replaced brake fluid using Pentosin Dot 4 with my pressure bleeder at 20 PSI. Bled each caliper using Porsche recommend order until no bubbles were present in the line. My thinking was that the car sat for long periods of time and the brake fluid became contaminated.

Result:
Slightly looser pedal feel than before with calipers still sticking.

Attempted Resolution No. 2:
Replaced master cylinder. I bench bled the new master cylinder with Pentosin Dot 4, installed and bled the brakes using my pressure bleeder at 20 PSI until no air bubbles were present.

Result:
The test drive revealed mushy brakes. I re-bled the brakes, removed the supply lines from the new master cylinder and re-bled the master cylinder to remove any possible air.

On my second test drive, the car was experiencing significant drag and mushy brakes. I raised the car to discover that all 4 calipers appeared to be engaged and causing major drag. I opened the bleeder nipple on all 4 wheels which allowed the calipers to disengage.

I performed tests to determine if the booster was operating normally and it appears that it is.

My questions are:

1. Why were the calipers engaged after pressure bleeding? IS this normal? I realize now I should have verified that the wheels turned normally before taking it off the lift.
2. Is it possible that there may still be air in the new master cylinder causing the mushy brakes?
3. Is it possible that the check valve has failed?

I've messed with this brake issue for 2 weeks now with no resolution

paulofto

I know it sounds unlikely but what are the chances that the actual brake caliper pistons are shot or at least the seals are rotted away? If the fluid was bad it may have damaged the pistons or seals leading to friction and seizing. It seems unlikely that all 4 would be affected this way.

PS. You might want to post this on 986forum.com in the tech section. Lots of activity there.

Macster

Well, I am no brake expert. Are you sure the brake fluid reservior cap vent is not blocked? If it is this can build up a bit of pressure in the reservior and this will cause the pads to drag. This heats up the pads (and rotors) and fluid and this can result in more pressure build up and more brake pressure.

Anker

The fact that the problem exists on all four wheels indicates that it can't be sticking caliper pistons.

Do you have PSM or ABS? If you do, it could be a problem with those systems.

But my bet would be on the booster.

extanker

how did you test the booster ? All 4 wheels dragging point to a maladjusted /bad booster return spring. With the engine running the vacuum over comes the internal [bad] spring and applies the brakes. As for mushy brakes if you did not have them before you messed with the master/bleeding......................................there you go

michael_j_brown

Thank you for the information I had not heard of that site I will check it out

michael_j_brown

I have discovered that turning on the engine and applying brakes causes the calipers to engage but not release. I can only get them to release by opening the bleeder valves, front and rear. Also, I disconnected the vacuum line from the booster while the calipers were engaged fully expecting them to release and pressure to escape. This did not happen. I hate to give up and have it towed to Porsche but I'm running out of options.

968workaholic

Look inside of the brake booster, I bet it's full of rust. Normally when that is the case you need a brake booster. It's caused by blocked drains by the battery, water gets into the back side of the booster where the linkage from the brake pedal comes in and destroys it. If you lift up on the pedal with your foot, it should release the brakes, just to verify the condition. As for the soft pedal, sounds like air in the system, but you will be bleeding it again if you have to replace the booster.

michael_j_brown

I will check this morning just for clarification, pulled a vacuum line from the front of the unit and inspect from that opening, correct?

from everything that I am learning it is definitely pointing to the booster. I have read that Porsche changed the linkage assembly on the rear of booster in December 1999 and all new boosters that are manufactured use the newer assembly, is this correct?

i'm having a difficult time locating the parts that are required and instructions for this work? If this has to be done, is this something I could do or should I just give up and send the car to Porsche?

I'm wondering if putting the new master cylinder made it worse because it's newer and probably has more pressure?

968workaholic

Yes, look through the vacuum line hole. Get a new booster and see what it comes with, the last few I have changed came with the new linkage adapters. It is time consuming, but not out of the scope of the average at home mechanic. The new master cylinder is probably more efficient at transferring fluid pressure, thus making your initial problem worse. I still think you have air trapped in the system, causing the soft pedal though

Anker

The air may be trapped in the ABS and/or PSM circuitry. If that's where it is, you will need to have it bled at a dealer or indie.

968workaholic

Even on the boxsters with traction control, there is not a section in the factory tester for bleeding the abs unit. The old fashion way of finding some "loose" ground to activate the abs is the best way to get the fluid moving through the secondary side of the system

Alex Sol

gents, i'm bringing up this old thread vs starting a new one as i think it's directly related

did the OP solve the issue? what was the problem?

I had same problem a few days ago...
driving down the highway about 60 miles per hour (minding my own business as usual)
had pulled out the 986S 2000 vintage after about six weeks of storage. I had driven a few times in the Toronto winter and was busy on a project so parked it... last drive was in March and was in a winter storm in about 4-6 inches of snow... made it home in one piece and did get a bunch of salt from the road salters that day...

fast forward to Wed night when I took out for short drive.. as i'm driving i lifted off the gas pedal and slows down considerably... gas it and it would go and felt like i was losing power so i down shift a few gears... same goofy feeling... quickly got off the highway and as i'm trying to get into the closest mall parking lot, i'm completely stopped. even in first gear i cannot move the car...

rear brakes glow red like formula one hairpin brake zone...

call auto club and wait about 20 min
decide to try to move the car and sure enough it moves without any braking force and we push it off the road... turns out my battery was also dead from winter storage!!

push started the car, idled for ten minutes, cancelled the auto club call, drove home with no problems...

after reading this thread, i have a plan
a. check the booster and related parts
b. bleed the brakes - i had done a brake bleed in summer of 2020 prior to a track day
c. pull the calipers and rebuild - i'm an expert at this.. i've done six calipers in my life
d. check pads, rotors and replace the brake wear indicator that caught on fire (yup) from all that heat build up..

Dllrd23

my experience on this is most likely brake hoses deteriorating internally due to age. They will cat like a check valve and not allow fluid to flow back when pedal pressure is released. It would however be somewhat odd if all 4 failed at once but at 22 years old they probably would be a good preventative measure.

Alex Sol

Okay, super info, i've never experienced a brake hose / line acting so crazy... I was doing same 55 mph and driving about 6-8 minutes and hadn't touched the brakes for about five minutes... and they clamped on me! And knew something wrong right away though not expecting such drama. Forgot to mention one fact. I had just pulled out of winter storage and was on an crappy harbor freight trickle charger. Car started up no problem after six weeks no driving. Last drive was in a snow storm so maybe some salt! And... when the brakes failed so did the battery or ... was the dying battery the cause? So i happened to have volt meter in the car and it read 10.6volts. We push started and rev'd up engine. Got 13.8volts. And did a AC reading and showed about .1 volts... so it's not the alternator according to my findings... drove home, no problem with brakes or battery. Turned off the car. Restarted just to check... haven't touched the boxster in a week...

if I'm doing new brake lines, then I'm going to rebuild the calipers... fresh seals, fresh fluid... maybe replace the piston if all pitted otherwise reuse... only 60,000 miles on this 22 yrs young boxster!!

I'll inspect this weekend, order new parts, check in.. where to buy? pelican, ebs racing, paragon, vertex or rockauto?