95 Toyota Landcruiser 80 Series Brake Pedal Sinking

[Sir Plug]

Help, my mate has an 80 series Landcruiser that has a sinking brake pedal. We replaced the master cylinder believing that to be the problem but it's still the same. We've clamped all the hoses and it still sinks and there is no fluid loss anywhere. What we have found is that the pedal only sinks when the engine is running. Also, as it sinks, a red warning light comes on the dash (circle with exclamation mark) no other warning lights are on and the ABS works as it should.....any ideas??

[ClassicCruiserSpares]

Sounds like its got air in the system somewere and if its got an abs unit they can be a prick to bleed the air out of some times.

[Heath]

Vacuum system in the booster. Sounds like a hole in the diaphragm. that's my guess.

[Sir Plug]

Vacuum system in the booster. Sounds like a hole in the diaphragm. that's my guess.

If it had a hole in the diaphragm, surely the pedal would be harder to press, as if the engine wasn't running?

I'm lost for an answer!

ClassicCruiserSpares, how do we get the air out the ABS system?? Does it need bleeding with the motor running?

[Sadam_Husain]

If it only sinks with the engine running it sounds like some sort of vaccum or booster problem, you can discount air in the system or a fluid leak then?

[Landy Rover]

Did you replace it with a new one or sec-hand? A pedal that sinks to the floor is fluid passing back from the presure seal and returning to the resivoir. Air in system will give a spongy or springy pedal. A leaking booster will act like its not working with a hard pedal.
If you did fit a sec-hand one its more than likely your problem.
Bear in mind with engine running a lot more force is placed on the master cylinder piston which if it is not in best of health can bleed down.

[Sir Plug]

Did you replace it with a new one or sec-hand? A pedal that sinks to the floor is fluid passing back from the presure seal and returning to the resivoir. Air in system will give a spongy or springy pedal. A leaking booster will act like its not working with a hard pedal.
If you did fit a sec-hand one its more than likely your problem.
Bear in mind with engine running a lot more force is placed on the master cylinder piston which if it is not in best of health can bleed down.

New cylinder...

[Sir Plug]

OK...all sorted. In the end, we had to have the engine running to bleed the brakes. Loads more air came out but now all is good.....must be something to do with the ABS module

[ClassicCruiserSpares]

Good to hear its all sorted, but yes the vehicle does need to be running to bleed the air from the abs.