Rear braking pressure regulating valve

[Etienne B.]

Hi all,
With all round new or rebuilt brakes (1964 Cooper S), and after approximately 1200 miles of driving,
I experience that the rear wheels do lock much earlier than the front ones ...
Can be useful ( ) but will definitely be embarrassing in case of an emergency !
any idea ?
I suspect I do not have the right rear brake pressure regulating valve : which one should it be ?
Cheers, Etienne

[Vegard]

What size rear cylinders do you have?

[Michael7]

The Cooper S used the 21A1774 proportioning valve, with an opening pressure of 350-400 psi. You could possibly have fitted the non-Cooper S valve 21A2031, which opened at a lower pressure of 290-340 psi, thereby operating the rear cylinders sooner than intended. Your rear cylinders should be GWC1101, which are .625" bore diameter, although later Cooper S models had GWC1102, which are .75" bore. Either of these should give satisfactory results.

I believe the only difference in the 2 proportioning valves is an internal spring, and the Cooper S used the 27H6620 spring, but no idea where to find one of these.

Hope this helps.

Michael
www.7ent.com

[WNX700H]

The valve is normally open the pressures quoted are the pressures at which the valve starts to close and halt rear line pressure increasing so higher the number the more braking on rear axle before the valve works .

[vulcanbb18]

Yep other way around as WNX700H says - the pressure limit is when the valve closes to stop extra pressure being applied to the back where it will get up to no good...

Etienne - as Vegard has asked, what size wheel cylinders on the back? I'd be saying you'd definitely want the 5/8" (0.625") cylinders with std 7.5" S discs.

cheers

Jacob

[360gts]

Anyone found a supplier for the spring that fits the 21A1774 valve???

[970s]

27H6620 Minispares parts!

Al