Brake bias valve

[Paul V8]

Hello, I have a triumph TR7 V8, I have fitted AP calipers with 305mm discs on the front, the rears are standard but will be discs soon, the problem i have is when i brake the pedal feels very hard and doesnt slow down like i think it should, the servo works ok, should i do anything to the bias valve, like throw it away, run a pipe from the master straight to the fronts and fit an adjustable valve in the rear line. Any ideas? Thanks.

[Ian Anderson]

Something sounds wrong!

What size Master cylinder are you running (bore diameter)?
What are the piston diameters in the calipers?
What pedal ratio?
When you stand on the brakes which end locks up first (if you can lock the up)?
The above will tell if you have a bad set up and something needs changed.

Does it make any difference if you remove the vac advance (plug th engine side) than drive?

Ian

[Paul V8]

As far as i can work out, the front calipers are 4 pot APs and the pistons are 38.1mm each, rear wheel cylinders are 18mm and the master cylinder bore is 22.2mm. Dont know the pedal ratio, standard TR7. I think i can get the rears to lock. Any ideas?

[Ian Anderson]

Presumably this is a tandem Master cylinder - where a single push rod from the pedal pushed into 1 cylinder

A better one would be a twin master cylinder (seperate cylinders) with a balance bar between.

If you can lock the rears first (drum) then you are not getting the correct brake force on the fronts.

I have similar diameter pistons on the Caliper but run twin 0.7 masters and the brakes on the GT40 are like a big hand grabbing hold of the car and slowing it down!

I would think that you are getting pretty close to no front brake at all.

First thing I would check is to pull the master apart and check seals and operation of it - possibly the seal for the front brake circuit is leaking back to resevoir and only the rear brake working. (buying a new one may work out cheaper and more reliable in the long run by the time you have stripped it and put it on and off a couple of times)

Ian

[davemgb]

Are you running braided brake lines? These make the pedal seem harder.

Also, what pads and shoes are you running - if they are uprated they might not work until hot.

Dave

[Paul B]

Hello, I have a triumph TR7 V8, I have fitted AP calipers with 305mm discs on the front, the rears are standard but will be discs soon, the problem i have is when i brake the pedal feels very hard and doesnt slow down like i think it should, the servo works ok, should i do anything to the bias valve, like throw it away, run a pipe from the master straight to the fronts and fit an adjustable valve in the rear line. Any ideas? Thanks.

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How do you know the servo is working okay? Try disconnecting the vacuum pipe and seeing if your brakes get a lot worse. You may find the servo is not delivering its full worth for some reason.

Try standing on the brakes when doing 20mph on a gravel track, with a witness to tell what the wheels do. This is a much easier way to test braking balance than doing it on tarmac at higher speed.