Boxster S 986 spongy brakes

[Tiberious0]

Hi!

I have this problem with the brakes on my 986 Boxster S (with PSM): the pedal feels spongy, there is no bite; almost like there is some air in the system.
- The ABS works - I can feel the grinding when braking hard.
- The braking power looks OK, so it stops the car.
- It is not the perception issue. I drove another Boxster S and the brakes there felt normal; also all the previous mechanics confirmed it.

I had braking fluid changed and brakes bled twice, also the front and rear brakes discs + pads were replaced. That didn't fix the issue. Now one very trusted independent Porsche specialist has done some diagnostics and is suggesting to replace the brake master cylinder and servo and if that doesn't fix the issue, then the ABS pump.

So could anyone comment on my questions please:
- Has anyone had a similar issue?
- Is there a simple way to check that the master cylender/servo is the cause?
- Do you think it is realistic to have a some type of proper diagnostics somewhere instead of going and simply replacing parts till the issue is gone?

P.S.: Another suggestion from another independent garage (which had a customer with brake issues similar to mine) was "We did the master cylinder and ABS pump but still had a spongy pedal, I think the fault was in the wiring but the customer didn't want me to proceed any further. I think something to do with the pressure sensor wiring maybe."

 

[Tiberious0]

Any thoughts from anyone? I am really desperate with the situation :/

 

[GrahamW]

A couple of queries. Was the pedal feeling soft before you had the fluid changed or since the change? Sometimes the technician is a little "lazy" when flushing out the system with the new fluid and doesn't bleed all of the nipples on the calipers. Have you checked this? It will be quite apparent if they have all been bled as the nipples will show signs of use. Secondly, sometimes with age the brake pipes corrode and develop small holes. Has this been checked? They are hidden from view behind the underside floor panels and may not be apparent to the naked eye or by spots on the garage floor.

Finally is the slave cylinder or the calipers leaking fluid?

 

[AndrewCS]

No experience of this but looking through the parts diagram I don`t see a brake sensor. There is a regulator (valve ?) between the master cylinder and the ABS unit ... that`s not mentioned by your previous investigations

I assume that the pedal linkage is OK, NB ... the switches 17 & 18, not sure what they are for ?

There are a few things ^ that would be easier ? to change / test than removing - replacing the brake booster / servo and ABS unit ...

Porsche Classic Genuine Parts Catalogue - Porsche Great Britain

 

[Tiberious0]

A couple of queries. Was the pedal feeling soft before you had the fluid changed or since the change? Sometimes the technician is a little "lazy" when flushing out the system with the new fluid and doesn't bleed all of the nipples on the calipers. Have you checked this? It will be quite apparent if they have all been bled as the nipples will show signs of use. Secondly, sometimes with age the brake pipes corrode and develop small holes. Has this been checked? They are hidden from view behind the underside floor panels and may not be apparent to the naked eye or by spots on the garage floor.

Finally is the slave cylinder or the calipers leaking fluid?

Thanks for replying, Graham!

>Was the pedal feeling soft before you had the fluid changed or since the change?

The issue was there before; I had the fluid changed 2 times at 2 different (and trusted) Porsche specialists that didn't have any impact.

>Secondly, sometimes with age the brake pipes corrode and develop small holes.

I think they should have checked this, but it is a good point! But I didn't see any effect at all after the fluid change, even immediately after it.

>Finally is the slave cylinder or the calipers leaking fluid?

I understand that I would see a drop in fluid level in the tank in this case, right?

 

[GrahamW]

A couple of queries. Was the pedal feeling soft before you had the fluid changed or since the change? Sometimes the technician is a little "lazy" when flushing out the system with the new fluid and doesn't bleed all of the nipples on the calipers. Have you checked this? It will be quite apparent if they have all been bled as the nipples will show signs of use. Secondly, sometimes with age the brake pipes corrode and develop small holes. Has this been checked? They are hidden from view behind the underside floor panels and may not be apparent to the naked eye or by spots on the garage floor.

Finally is the slave cylinder or the calipers leaking fluid?

Thanks for replying, Graham!

>Was the pedal feeling soft before you had the fluid changed or since the change?

The issue was there before; I had the fluid changed 2 times at 2 different (and trusted) Porsche specialists that didn't have any impact.

Are you sure all the nipples have been bled?

>Secondly, sometimes with age the brake pipes corrode and develop small holes.

I think they should have checked this, but it is a good point! But I didn't see any effect at all after the fluid change, even immediately after it.

They would had had to remove the panels to check, did they?

>Finally is the slave cylinder or the calipers leaking fluid?

I understand that I would see a drop in fluid level in the tank in this case, right?

Correct, but maybe not as much as you'd think

If the above are all OK then I'd start with replacing the Master cylinder as the rubbers do/can wear over time.

 

[Tiberious0]

No experience of this but looking through the parts diagram I don`t see a brake sensor. There is a regulator (valve ?) between the master cylinder and the ABS unit ... that`s not mentioned by your previous investigations

I assume that the pedal linkage is OK, NB ... the switches 17 & 18, not sure what they are for ?

There are a few things ^ that would be easier ? to change / test than removing - replacing the brake booster / servo and ABS unit ...

Porsche Classic Genuine Parts Catalogue - Porsche Great Britain

>There are a few things ^ that would be easier ? to change / test than removing - replacing the brake booster / servo and ABS unit ...

Thanks! Yes, that sounds exactly like the possibilities I would want to explore; in theory I would come to some garage with the list of those and ask to check those.

 

[Tiberious0]

A couple of queries. Was the pedal feeling soft before you had the fluid changed or since the change? Sometimes the technician is a little "lazy" when flushing out the system with the new fluid and doesn't bleed all of the nipples on the calipers. Have you checked this? It will be quite apparent if they have all been bled as the nipples will show signs of use. Secondly, sometimes with age the brake pipes corrode and develop small holes. Has this been checked? They are hidden from view behind the underside floor panels and may not be apparent to the naked eye or by spots on the garage floor.

Finally is the slave cylinder or the calipers leaking fluid?

Thanks for replying, Graham!

>Was the pedal feeling soft before you had the fluid changed or since the change?

The issue was there before; I had the fluid changed 2 times at 2 different (and trusted) Porsche specialists that didn't have any impact.

Are you sure all the nipples have been bled?

>Secondly, sometimes with age the brake pipes corrode and develop small holes.

I think they should have checked this, but it is a good point! But I didn't see any effect at all after the fluid change, even immediately after it.

They would had had to remove the panels to check, did they?

>Finally is the slave cylinder or the calipers leaking fluid?

I understand that I would see a drop in fluid level in the tank in this case, right?

Correct, but maybe not as much as you'd think

If the above are all OK then I'd start with replacing the Master cylinder as the rubbers do/can wear over time.

> Are you sure all the nipples have been bled?

Jaz Porsche (https://jazweb.co.uk/) did the fluid change - if they didn't do it right, I don't know which indie to trust then :/

> If the above are all OK then I'd start with replacing the Master cylinder as the rubbers do/can wear over time.

I see! Do you think there is any definitive way to diagnose it, or at least some specific signs that are related to MC - rather than this being the next most likely thing?

 

[Motorhead]

All of the above Artem. Finding the source of spongy brakes can be a bit of a rabbit hole! A faulty master cylinder or ABS unit could be the problem, and as Graham has said, techs aren’t always diligent when it comes to bleeding both nipples on the calipers (assuming there are inner and outer nipples on your 986?). Watch out though if you’re planning to check the inner nipples as it’s most likely that they’ll shear off if they’ve never been cracked open.

Good luck!

Jeff

PS: The 2-switches 17 & 18 that Andrew mentions are purely to monitor brake and clutch operation (the latter to ensure that the clutch is depressed before starting the engine) and shouldn’t (won’t?) have any bearing on your problem.

 

[Tiberious0]

Great thanks everyone for the previous suggestions! Let me follow up on my original problem with the spongy pedal; in short, the garage fixed the problem, but their solution caused PSM light to appear.

The garage:
- Replaced the brake master cylinder and servo.
- Bleed the brakes. In the process one of the nipples stripped its thread, so they fixed it.
- Replaced the ABS pump with a used one.

That had fixed the issue with the spongy pedal! I was about to collect the car when they called me about a new problem - with PSM: "Error: 4460: pressure sensor fault implausible value".

They shared that the issue didn't always appear immediately and could pop up after driving the car for some time (once they drove it for 10 miles before the light came again). What they tried and replaced:
- brake pedal switch
- MAF sensor
- Another ABS pump
- That another ABS pump, but with the electronics/brain from my original pump.
- PSM pressure switch (Bosch)
- Electric boost pump
- Checked all the wiring between the pump and the switch for short circuits
- Instrument cluster

The car is still in the workshop; things that they yet are about to check:
- Speed sensor on the wheels.
- steering angle sensors - check the function of it.

They are saying they have tried everything they could think of and are suggesting to convert the car to be PSM-less.

Does anyone have any ideas what else it could be? E.g. could it be that something went wrong with the brake cylender/servo - as the problem first occured after the garage replaced those (+ABS pump)?

I would really appreciate any advice please as I would hate to sell the car - I need PSM for safety reasons and lack of driving skills

 

[Motorhead]

Hi,

I’m not sure if this is relevant but I note that the brake pedal switch has been replaced, but has it been adjusted correctly? According to an appropriate post on RennTech:

NOTE: A malfunction of the stop light switch can cause this fault. The stop light switch must have switched before a brake pressure is recognized. When the brake pedal is actuated, the stop light switch and the brake lights are triggered. The brake pressure on the pressure sensor is 0 bar.

I take that as meaning that the switch needs to be positioned to activate during the brake pedal’s free-play?

Jeff

 

[BrianJ]

I certainly wouldn't go PSM-less. PSM used to be an option on early 986 and I knew several people who had entertaining crashes at really low speeds starting from rest and turning corners on wet roads on PSM-less cars.

 

[Tiberious0]

Thanks for the replies!

The key thing is that I've never had this PSM error before and it only occured after they replaced the ABS pump (they've tried two diferent ones), brake cylender and servo. Could those actions have been done in theory incorrectly so that caused the issue problem?

> I’m not sure if this is relevant but I note that the brake pedal switch has been replaced, but has it been adjusted correctly?
I will check! Does the switch need to be adjusted after the replacing the parts mentioned above?

> I certainly wouldn't go PSM-less.
Yes, me too, but they the garage has almost exhausted all the options to fix that, hence me trying to get some advice here :/

 

[Motorhead]

I wouldn’t think that replacing those parts would require the switch to be adjusted, but since the switch was replaced it needs to be positioned correctly. Just check that the brake lights come on immediately the pedal is depressed [during the free-pedal movement] and before the brakes are activated.

Replacing the ABS pump, etc., will have required a diagnostics hook-up to activate the ABS and purge the system completely of air, but any garage worth their salt will have done this, a Porsche specialist in particular.

Jeff

 

[marc_tt]

Had a similar problem with a VW and turned out to be tiny split in vacuum pipe. I Don't know if 986 has similar vacuum pipe.

Had a 986 brake pedal switch fault that showed up when using cruise control and first replaced switch with aftermarket switch that produced new faults and switch was not adjustable. Then got official Porsche switch from OPC and everything worked fine after fitting official Porsch switch.

Hope that helps

 

[JohnCRS]

Hi Teberious

I note that there is a two year gap between your original query and the thread resurrection on here. Has your problem been investigated for that long a period? The reason I ask is what is the state of the battery. I had a PSM fault come up on my 981 and is was the first indication that I had a voltage problem. A fully charged battery resolved the problem - well that plus a full set of coil packs and a valve cam solenoid, but no need to go there. PSM was fine.

 

[Tiberious0]

Thanks, everyone; I am collecting all possible ideas so I could pass them to the garage and they could check everything at once!

> . Just check that the brake lights come on immediately the pedal is depressed [during the free-pedal movement] and before the brakes are activated.

And to follow up on this, apart from the pedal switch, do you think it is possible that something wasn't right even before they replaced those items, so the PSM issue just happened to manifest itself afterwards? I still think it is strange that my car didn't have this error ever before.

> I note that there is a two year gap between your original query and the thread resurrection on here. Has your problem been investigated for that long a period?

Not really, last two years were quite busy, so I got the car to the garage last year. But thanks for the tip on the battery!

> Had a 986 brake pedal switch fault that showed up when using cruise control and first replaced switch with aftermarket switch that produced new faults and switch was not adjustable. Then got official Porsche switch from OPC and everything worked fine after fitting official Porsch switch.

Was it the same code I had - Error: 4460: pressure sensor fault implausible value?