Hard brakes

[renault12ts]

Having fixed my wheel sensor problem, I want to now address another one.

If I use the brakes several times in quick succession, the pedal goes very hard. I presume the reservoir of vacuum is being used up and I am losing assistance, but why? I would have thought that it should be able to stand this sort of use.

 

[BlackC55]

Is this while driving or stationary?

 

[renault12ts]

While driving. Usually apparent first thing when moving off if I use the brakes a couple of times in quick succession whilst negotiating my driveway on to the main road.

But it happened tonight when I was testing my brakes following the wheel sensor fix.

 

[nigel cross]

When did you last had your brakes bleed or the fluid changed?

 

[renault12ts]

Fluid is fine. Level is fine

 

[Number_Cruncher]

Taking the usual risks with this topic!, one thing that might be worth checking is front wheel bearing play - the disc can move, and knock back the pistons, which can make the pedal feel artificially soft, but, a few quick presses can give the impression of the pedal becoming more solid.

 

[nigel cross]

Fluid is fine. Level is fine

That is not what i asked

 

[renault12ts]

That is not what i asked

Sorry, last year.

 

[colin_397]

I'd just check for leaks in the vacuum system. I had a similar problem on my discovery, and I found a small vac pipe running through the bulkhead had a hole in it. Therefore loosing vacuum. It was only noticeable at idle when parking etc. Mainly when parking at work!! I reversed into a fence 2 days in a row!! Lol

 

[jefrs]

Depends how quick your 'quick succession' is.
If very quick then vacuum, if not then brake heat:-

But note - Pumping the brakes should make them go harder if there is air in them i.e. they are normally spongy, which they should not be.

Brakes get hot, air in system expands, brakes go hard.

Level looks normal but needs bleeding, best changed.

Problem can also be parking brake binding, drum gets hot, disc gets hot, air in fluid expands etc etc.