Lizard928

I have been noticing more and more failures when it comes to the 928 S4+ brake master cylinders.

A customer of mine just got a new master cylinder from one of our reputable parts carriers. And I am sure that he will be looked after.
But it seems that more and more are going south.

The failures are all the same with the pedal being pushed pressure builds and then slowly sinks down to the floor or nearly the floor.

However this same owner had a local shop tell him the ABS pump was needing to be replaced. I have noticed some wetness on one of the lower two bolts/nuts (forward). But there was oil all over the place in there and it's impossible to see if it is actually leaking from there right now.
Has anyone seen an ABS pump leak?
Car is question is a ROW 90 S4

James Bailey

I have not seen or heard of any ABS unit problems but the cars are getting older. The unit only decreases the pressure to the wheels which it thinks are locking up never is it a pump to build pressure.

Mrmerlin

Colin the only time i have seen an ABS pump leaking was when the brake proportioning valve was loose

James Bailey

I think Porsche way over engineered and tested that system because of the potential liability.

Lizard928

I have had the same experience with the ABS pumps (understanding it is not a pump ).

I just wanted to confirm on here.

That said, popping off the wheel lining, and cover to get a proper look at it will be a good idea.

jon928se

Just a thought - it could be an internal leak in the ABS unit - for example at the solenoid operated valve/s that release pressure to the brake circuits allowing pressure to dissipate into the internal reservoir.

Lizard928

Interesting thought Jon. I will lkeep that in mind.
More cleaning in the morning.

WallyP

Bypassing (a slowly sinking brake pedal, usually under moderate pressure) is common when a brake system gets old. The two main causes are hardened/worn piston seals in the master cylinder, and wear or corrosion making the master cylinder bore rough.

Changing the brake fluid every couple of years helps stave off this problem...

AO

It's a closed system... so that cannot be the problem.

Lizard928

Wally,

I concur with you on this. And this is a new master cylinder on this particular car.

jon928se

You're correct it is a closed system, however the design of the system is such that a leak at the solenoid operated valve would allow fluid to flow into the spring loaded reservoir in the ABS unit. When pedal pressure is removed this fluid would then flow through the pump chamber back to the MC.

jon928se

And corrosion in the MC could allow particles of rust to flow through into the ABS unit thus damaging the seals in there as well. Even if there is no corrosion in the ABS unit - it's steel so that can corrode and damage seals as well.

It's also worth noting that the damage from corrosion in the MC bore usually first happens when bleeding or flushing the brakes as it's normally the only time that the Pistons in the MC travel through there full range of movement. Moral of this story is: If flushing very old fluid out of a system that has been unserviced for a long time, remove the MC and inspect/clean/replace seals or whole MC first, to avoid the rust being sent down the pipes to the ABS unit and calipers.

Lizard928

Update to this.

This is a lower mileage 90 S4 and the ABS pump is indeed leaking fluid off the bottom two fittings. More from the front than the rear.
There are the two nuts on the bottom of the unit.

Pump part number 92835575505 Bosch pn 0 265 200 046
serial 061/551429

I am going to see if I can source one locally.

I am also going to run a test. I have a 87 ABS pump that is physically identical.
The relays are different with one having more pin outs.
The internal wiring is slightly different too.
Except the wires going to the solinoids are the same with the same pins.
I have removed the electrical off the 90 pump. And have installed it onto the 87 pump.
The only change needed was to cut a form of a diode w/ pin off the end of one solinoid.

I will let you know if this works. If it does it has saved the owner some funds and down time. If not then I am out probably around an hour of labour.

jon928se

Colin

my reading of the ABS Tech info suggests that the 87+ ABS units are mechanically all the same. The changes that were made all seem to be to do with voltage limiting Zener diodes being moved around between differing styles of ABS relay, and additional wiring/pins on relays for cars with PSD. So hopefully you should be good to go if you've got the electrical side matched up.

AO

Ok... So it's a closed system... Unless it leaks.