Master Cylinder pump
 

Hi guys

I've been having a problem that might be related to the Master Cylinder on my 2wd Saph Cossie.

When I touch the brake pedal, even if the car is parked, I get a small thump from the pedal. When the car warms up it almost disappears. Also, when I turn the key from cold, the master cylinder takes a long time to build pressure. I can hear it working for 10 seconds or more (it didn't do that for that long). Other than that never had any problem with the brakes and the car brakes fine.

What could be wrong here?

Thanks

Check the Earth and power to the pump. Old Cossies often need the plug between the bulkhead replacing.

If not then replace the pump. I have one from a 4x4 if they are the same although getting it to Portugal could cost a bit.

Thanks mate, Will check that out. Just realised I haven't changed the brake fluid in like 12 years... That will not help for sure. I'll start with that. Just realised it takes way longer than I thought... It runs for over a minute when I start it from cold.

Is it earthed through the loom or a strap/wire to the chassis somewhere?

I am chasing a similar problem. 95% of the rime my brakes are perfect. Nice solid pedal. Then from time to time the pedal is really soft and goes to the floor and i have to pump it up before i hit something.

Scary.

It does seem to happen when i am doing full lock 3 point turns etc when i hit the brakes its soft. But then is fine.

So definitely not bleeding brakes. The ABS light has been on for about 2 years but its not the failed sensor. If anything i can only track it down to since i removed/reinstalled the engine and sprayed the bay so an earth problem may explain it. Hell the silly slutch cable tenaioning up could be affecting the earth...obviously guessing here

I think the only earth is through the loom, might not hurt to earth it to the chassis too.

If the pump went off though would it not give a hard pedal and not soft as it is trying to do the same as a vacuum?

I need to get my head around how the Sierras work. It feels as though in a traditional vacuum assisted setup that the check valve is playing up. How that equates to the Sierras setup. Well thats what i need to figure out.

So traditionally the pump flogging out gives a hard pedal?

That is only an assumption, it may be the other way around. See if the earth changes it.

I do have 2 spare pumps here in the UK, they sell for around £100 each normally.

If by pump you mean the ABS system then there are three components to the ABS syste, - Actuator, pump and valve block (plus ECU). The pump is the one with the black ball shaped unit that is used to retain the pressure. On switch on the pumps pumps fluid until it reaches the pressure. This can take up to 20+ seconds. Over time the nitrogen in the sphere leaks and it will take longer so over 30 and you need to chek things out. Also when you break, the pump pumps up the pressure again (obviously).



Spheres are no longer available but I tracked down a guy in the USA that can repair them via a mod. They were used on Volvo and a good many other cars.


I also have a series of diagnostic manuals to check out the system. My ABS light stays on now and I currently do nt know why even though it all checks out OK.

Hi,
There's a pressure switch in there that controls the pump via a relay.
If I remember correctly this is independent of the rest of the system, as in you can remove the abs ECU and the pump still works.
If the pump runs for a long time it's either not making enough pressure or the switch isn't working properly. I think the sphere is a "pressure store" Be careful there's a lot of pressure in there!
When we tested one with a pressure gauge it straightened out the hose when the pump ran up to pressure!
Cheers :)

Hi,
Just pump the pedal with the ignition key out you should be left with a hard pedal iirc.
The pump kind of replaces the servo but the difference is the brake fluid is pressurised to feed the master cylinder, which is just a valve that controls the fluid going to the calipers. The pedal is directly connected to that valve.
Cheers :)

Fluid seems the best place to start, do not under any circumstances mess with the accumulator, which is what Gavin describes (unless you have depressurised the system).

Richard, I assume you have checked the wheel sensors, as this is typically the problem?

Martin

The pump is fed from a relay triggered when the ignition switch is on first position (and also by second position when engine is running).


Yes, you could agrue the pump is not part of the ABS but the ABS ECU will not say all is OK and turn out the dash lights unless the pressure is up (and some other checks pass too). The pump pressurises the fluid one side of the sphere against nitrogen on the other to maintain 2,000 psi. The pump should stop after 30 or so seconds on cold start up and less than a minute. If not, the pressure switch could have failed or the sphere may have. You cannot buy new spheres now - I found ne guy in USA who can repair them.



When one presses the brake pedal, the actuator (a big master cylinder) lets fluid to the calipers via the valve block. If the ECU detects locked wheels, it pulsates the valve blovk solenoids and you have ABS.


The wheel sensors are a common failure and will cause the ECU to fail its self-checks and you will not have ABS - yellow lights on. If pump OK, you will have brakes, though. If pump does not reach pressure, you will not have any brakes - red light on.


My binding issue is not sensors as they only effect the ABS pulsating. I have just sent all calipers away for refurb. If I still have binding issues, it can only mean a failed actuator - unheard of but could be. Having previously changed pump, actuator and ECU with no change, I hope my problem is calipers.

Hi,
Have you checked for play in your wheel bearings?
Cheers :)