Boost in Servo?

[DPJ]

I have a good normal brake pedal nearly all the time. I'm now finding however that on sustained boost (ie a power run) where it's at 1.4 bar+ for a few seconds, my brake pedal goes hard right at the top. If I brake suddenly, the pedal resists for a fraction of a second before vac cuts in.

Has anyone else experienced this?

I haven't changed anything on my manifold to servo line. I'm guessing boost is leaking past the (two?) one way valve(s)?

I suppose the remedy would be to try some new valves.

 

[barnsley]

never had and trouble my self i would replace them

 

[h17och]

im sure that is just what happens on turbo cars. because a servo works on vaccum...

no vaccum = no servo

 

[DPJ]

im sure that is just what happens on turbo cars. because a servo works on vaccum...

no vaccum = no servo

Vacuum's fine and works the servo Hooch. I mean I think I'm getting positive pressure in the servo.

 

[h17och]

yea but what i mean is if your on boost then there is no vaccum in the servo.

no vaccum = no servo

its not untill the engine goes into negative pressure then the servo will work

i have that problem too (if im understanding it right)

basicly i get it if im horsing it along then sum granny pulls out on me ill have about 1second where the brakes do nothing then all of a sudden they come in really hard. its because i get me foot on the brake pedal before the pressure is neg.

i maybe wrong but thats what ive always thought!?

maybe mine is aproblem too

 

[DPJ]

yea but what i mean is if your on boost then there is no vaccum in the servo.

no vaccum = no servo

its not untill the engine goes into negative pressure then the servo will work

i have that problem too (if im understanding it right)

basicly i get it if im horsing it along then sum granny pulls out on me ill have about 1second where the brakes do nothing then all of a sudden they come in really hard. its because i get me foot on the brake pedal before the pressure is neg.

i maybe wrong but thats what ive always thought!?

maybe mine is aproblem too

As I understand it, any time the inlet manifold has vacuum, it will create that vacuum in both chambers of the servo. The non-return valve prevents that vacuum from being lost. When you brake, an amount of air (dependant on braking effort) goes into one servo chamber and the pressure differential between the two chambers gives the brake assistance.

When my engine isn't running, my brake pedal is hard at the top. When I switch the engine on, I lose this hardness at the top - that's the effect of the vacuum. It's the same on all the (petrol) cars I drive.

My point is that my brake system works normally nearly all the time - a tad of give at the top of the pedal. The exception is when I've been running under full boost for a few seconds. If I then come off the gas and onto the brake, my pedal is hard at the top.

I can only attribute this to the servo getting pressurised air in it.

If I touch the pedal at this point but hardly brake, I can feel the effect of the new vacuum removing the top-pedal hardness.