Brake issue

[Taff107]

Excuse my poor mechanical 'nouse' but I have an issue with the brakes.

Engine off - brake pedal solid- hard to move and no brakes.
Start the engine - brakes seem fine.

Does this suggests the servo is fine and the problem is with the master cylinder?
Fluid levels are fine

[TonyRPH]

I would expect the brake pedal to be very firm when the engine is not running, but the brakes should still work - the pedal will just require a *lot* more effort.

If the brakes work fine with the engine running, then the servo is doing it's job.

Sometimes there is some residual vacuum and you will be able to depress the pedal with ease once or twice after the engine has been switched off.

If you hold your foot on the brake when starting the car, the pedal will also likely go down while doing so, as the servo assistance activates.

[Pica-Pica]

Sounds normal, I assume braking on the move is as normal?

I guess people did not notice the servo coming in years ago.
Now, most cars you have to start, as follows,
Manual transmission - clutch depressed.
Auto transmission- foot on brake.
It is the latter that you notice the effect you feel:-

• Foot on brake (so no servo), brake feels hard (because their is no servo assistance)
• Start engine, servo is activated and the servo assistance ‘pulls’ the pedal down further.

(‘Nous’ not ‘nouse’ )

[E-bmw]

Taff107 said:

Engine off - brake pedal solid- hard to move and no brakes.
Start the engine - brakes seem fine.

You have just described a normal brake set-up working correctly, with one caveat.

When there is no vacuum assistance (engine off) you are trying to move a column of hydraulic fluid that is dead-headed against the brake disc, so it needs a big push to do anything & feels solid.

When there is vacuum assistance (engine running) it is very much easier.

This is why when you push the brake pedal & it is solid, then start the car with your foot on the pedal, the pedal sinks down a bit more.

The caveat is that the vacuum that is there when you stop the engine would expect to still be there in part while the engine is off, so you MAY, I stress MAY have a slight vacuum leak when the engine is stopped.

Try this.

With the engine running feel what effort the pedal takes when pushing down on it.

Then stop the engine & immediately do the same a few times.

If the effort gets much harder after a couple of goes that sounds normal.

Restart the engine for a minute & check it is easy again.

Stop the engine, wait 10 mins & then try again without the engine running.

Same as before?

If so, do it again & leave it an hour, then try again.

If it is still the same then you don't have a problem.