Hi,

I have a brake light pressure switch threaded into my master cylinder, the problem is I have to press the brakes very hard before the lights come on. I have tried a new pressure switch but its still the same.

Any ideas

Thanks

Steve

is the switch above or below the pipe? ie - could there be air trapped between the switch and the fluid?

what are the numbers on the switch - i've been told you can get two pressures.

Its above the pipe, I do have a bleed nipple on top of the master cylinder so wouldnt have thought air could be trapped, but its a good idea and as it should be easy to swap I will try that tomorrow...eh I mean today, sh!t its late. I will check the number on it tomo..today too.

Cheers

Steve

My switch is mounted sideways and I too have to press the brakes quite a bit to get mine to light. I think on heavier cars, it causes no problem. But on out lightweight cars, they stop too well for the switch.

--Chris

MInes mounted sideways above the level of the m/cyl. The lights come on with only a slight press of the pedal.

Try bleeding it first.

come on Mike!!!

bleeding it will make no "bleeding" difference as the pressure will be the same whether its air or fluid in there!!!

bleeding it will make no "bleeding" difference as the pressure will be the same whether its air or fluid in there!!!

Fluid is incompessible, air is compressible. If the pressure were the same with air and fluid brakes would not need bleeding. If the switch needs a certain pressure to operate it I think entrapped air is a reasonable explanation as to why there is insufficient pressure.

I will of course be happy to be proved wrong.

John

Are you saying that the switch is measuring the pressure in the Master cylinder? - If so this may be the problem as the switch should measure the line pressure which is some 10 times higher.

A good M/C pressure is about 1000PSI - Line pressure can be 10-20,000 PSI - The Brake switches are normally fitted in a small bore union in the line to the front brakes or in a tee to the rears - this may also be why they have two pressures.

NSDEV is right - pressure is pressure air or fluid - one just takes less pedal distance to achieve as it does not compress - but the pressure is the same only the volume is different.

Cheers

Andrew

Do you have one master cylinder or 2 + bias bar. I had a similar problem as the rear brakes had so little pressure when adjusted to meet SVA that they wouldn't turn the lights on till I was braking V.heavy. I added an additional switch on the front circuit and that fixed it.

quote:

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Originally posted by John Bonnett
bleeding it will make no "bleeding" difference as the pressure will be the same whether its air or fluid in there!!!

Fluid is incompessible, air is compressible. If the pressure were the same with air and fluid brakes would not need bleeding.

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However, the air and fluid are both subject to the same pressure, a system can not magicaly have the air at one pressure and the fluid at another!

I have no argument with that. However, I do disagree that bleeding will make no difference. I think removing the air could solve the problem.

John

If its in the front of the system i'll agree with NS Dev, i'll agree with Nat. If its in the rear (and this is pre-sva) i'd say try bleeding it. The rears do a lot less work, any air will reduce there effectiveness so you need to stamp on the brakes before it does anything.

Its my theory, i'm probably wrong but i'm sticking to it until i'm told it didn't make a difference.

I have a similar thing on mine, its basically since i adjusted the rear bias a bucket load to make sure it got through SVA, i noticed i had to really jump on the pedal to get the lights on.

Give the brakes a bleed just in case, if that doesnt cure it, try putting it on the front line.

HTH

David