Brake Master Cylinder Upgrades

[dougie]

Hello- I need help to increase the pedal pressure, "make it firmer" on my '57
Austin-Healey 100-6 MM. This vintage racer has 4-wheel discs. and currently a 5/8 Girling brake master cylinder with the reservoir extension. I have bled it 3-4 times and bleed it after every race weekend but the pedal usually takes 1 strong half pump before I feel full pressure, hard to heel-toe in the heat of battle like this. I have a .75 Girling master cylinder on the bench I could change to, or should I look at Tilton's larger master cylinders .81-1.00? Also, does anyone race with the brake servo? I don't need the added weight if I can get the same results with the correct master cylinder set-up .

 

[JerryB]

Sounds like caliper pistons are retracting an inordiate amount.
Rotors wobbly causing knock back of pistons?

I assume you have a 5/8" front and a 5/8 rear master? with a balance bar?

Well maybe not...if only one 5/8" master you should be making changes.

Caliper piston diameter and pedal ratio + above info might help solve the equation.

 

[RonMacPherson]

Let me see if I understand your situation.

You have a single 5/8" master cylinder working both front and rear disc brakes?

If that's the case I can understand why you have to "pump" the pedal...

As the other poster suggests, ideal situation would be separate cylinders for front and rear with a balance bar.

Otherwise you're gonna be doing some engineer work(both hydraulic) and mechanical(pedal/rod) angle.

 

[billspohn]

I think Jerry is right. Assuming you have no problem with having a nice hard pedal after that first pump, you are no doubt experiencing pad kick-back from excessive end float on the rear (and possibly front) axle.

This is a well known problem on some 4 wheel disc cars, and you have to ask yourself if that rear disc conversion was really worth the bother. Early Jensen 541 and MGA Twin Cams deal with it - I always hit the pedal to bring the pads out with my left foot before hitting the braking markers in my Twin Cam race car.

You can make the best of it by minimising bearing end float, live with it, or convert back to the very decent rear brake set up.

 

[JerryB]

and really there is too many "possibles" that need answers before an absolute diagnosis can take place.

So if dougie needs answers .......

 

[dougie]

Further discussions with my mates in the UK have revealed that this common problem can be solved with a dual circuit balance bar pedal box utilizing a 3/4 master cylinder for the front and a 5/8 mc for the rear.

Thanks for the all the help,

Doug