Change from servo to non servo
Turbo Mini Forums

Im looking to lose the servo powered master cylinder to free up some under bonnet space.

I have found a master cylinder (duel type) in an old B reg scrapper.

Im using 4 pots up front and basically my question is will i need to change my rear cylinders for ones of a different size to keep everything working as it should?

Cheers Guys

i dont know if this is right, but my thinking on the matter is, if you do end up with too much braking at the rear, then why not simply fit a in line bias valve? seems to be ideal solution to me, a helluva lot easier, albeit a bit more expensive.

then again theyre could be a really good reason why that wouldnt work!

And on the 7th day........... God created turbochargers!

Dan make bloody sure the master cylinder is good! Personally I would never trust something as important as a brake master cylinder out of an old scrap car.........it might work now, but how long till it gives out??? At the MINIMUM strip it out and rebuild it with new seals! Think about it buddy, safety first and all that. They're not that expensive new compared to your life?

Anyhow, onto the question, the master cylinder you describe should work fine with the rear wheel cylinders. I am assuming you are still using the "H" shaped valve on the bulkhead? As this does the limiting to the rear brakes on the later cars, as well as shutting off one of the twin circuits if you lose a lot of fluid.
Or as Pete pointed out an inline brake bias valve will work a treat, and allow you to get rid of the aforementioned "H" shape valve, and also allow you plenty of fun left-foot braking......

PS While your at the scrappers don't forget you need the brake pedal off the car you get the master cyl off too.

Edited by Tom Fenton on 12th Jun, 2005.

im trying to think this one out, to reduce the rear locking up when you stamp on the pedal you want larger rear cylinders?

dan are you still selling the car?

I think it's 1/2" rears for discs and 3/4" rears for drums

Metro turbo weekend driver,Mini turbo in the making again!

yes you did!

thats where i got it from, didnt you work out the smaller the bore the higher the pressure?

Thanks for the advice!

Greypride - i dont really know what im doing with it right now, desparatly trying to get some summer work but thats just not happening, so i dont know....

Nutter Driver - interesting idea, I think ill fit the master cylinder see what its like, then maybe put a bias valve on.

Tom - Of course I'd rebuild the mater cylinder whadda ya think i am eh?
Cheers for mentioning that i would need a different pedal - i didnt know they were different!

daa daa: http://www.turbominis.co.uk/forums/index.php?p=vt&tid=4647

tom said:
"I always get this bloody wrong.......I'll work it out for me as much as anyone else.......

If caliper total area is 4sq", and master cylinder is 0.75sq" (simplified) then 50lbs force on 0.75" at the master cyl equates to 266.6lbs force at the caliper. (Pressure = Force/Area)
(Pressure created at Master Cyl = (50/0.75) = 66.6lb/sqin.

At caliper end, braking force = Line Pressure*Area = (66.6*4) = 266.6lbs Force)

If master cyl size is 1.5sq" (twice the above) then force at the caliper drops to 133.3lbs.

Therefore force with a master cylinder twice the size required to give the same caliper force = twice as much, e.g. 100lbs required with the larger master cyl to give the same 266.6lbs at the calipers."

Grey- can you post me a link to that thread if you know where it is? (D'OH beaten to it, cheers mate!)

Dan- the earlier brake pedal uses a clevis and split pin, the same as the clutch. The later servo type one has an integral 'rod' to connect up to the servo.

Edited by Tom Fenton on 12th Jun, 2005.

for certain i would see what happens when you change the master cylinder, the bias valve idea was more of a way to save you changing the wheel cylinders. i changed to front discs on my 76 car and was told i would need to change the rear cylinders the maintain the balance, i never bothered, its always been fine!!!

And on the 7th day........... God created turbochargers!