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SP250 Restrictor or non-return valve for brakes

Posts: 823
Joined: Sun Feb 14, 2016 11:15 pm
Location: Aberystwyth Wales

Re: SP250 Restrictor or non-return valve for brakes

Post by Sydsmith » Mon Jun 03, 2019 11:17 am

Can't comment on how or if the thing works or not but when I did my rebuild, I took the restrictor to bits and cleaned it out, it was full of gunge, it is a very simple job.

I guess it worked better after it was cleaned, but I could not tell the difference, it certainly had no effect on the double pump issue which, like heavy steering, I consider part of the thing about driving a 60's car. Syd

Ian Slade
Posts: 471
Joined: Sat Feb 13, 2016 9:54 am
Location: Akrotiri Cyprus

Re: SP250 Restrictor or non-return valve for brakes

Post by Ian Slade » Thu Jun 06, 2019 6:52 am

The restrictor valve is what its name suggests, it is there to reduce the flow to the rear brakes to stop them locking whilst under normal braking, nothing to do with the double pump which is caused by mechanical issues, ( bearings, disc set up and the rear axle flexing) it is interesting that Geoff ( Fossil) doesn't have the problem as he also has one of the modified 1970's axles. The fitting of a servo may require a higher restrictor value as there will be more fluid pressure at the rear brakes during braking due to the servo effect. Having said all that a clogged restrictor valve can cause a failure to allow an effective bleed which may cause a double pump under extreme driving but in these days never on the road as most are not using second and third at full throttle with rpm between 5.5k and 6.5K which really causes the axle to flex on twisty roads :shock:
Owner since the 70's, Genghis is slightly to my left.

Posts: 28
Joined: Thu Oct 27, 2016 3:36 pm
Location: Haslemere, UK

Re: SP250 Restrictor or non-return valve for brakes

Post by DaveM » Thu Jun 06, 2019 8:51 am

In this instance the valve being referred to is a "residual pressure" valve and not a "restrictor" valve. Restrictor valves are as you rightly say used mainly to apply artificial in-balance to the braking forces acting between the rear and front brakes. The residential pressure valve however maintains an amount of pressure, generally 2 psi on discs and 10 psi on drums, within the system after braking to keep the pads on the discs reducing the amount of fluids required to push them in on the next application. Disc wobble, loose bearings and the like can cause the pads to be pushed back and ride well clear of the disc surface resulting in the need for more fluid to be displaced to bring them back into contact. As seen on the darts master brake cylinder the 3/4" bore is right on the limit of being able to supply the required volume in one push of the pedal. Mounting the residential pressure valve as close to the master cylinder as possible helps retain a maximised buffer of trapped pressurised fluid within the system helping to push the pads back onto the discs in-between brake applications. However, with only a 2 psi valve installed, any in balance in the bearings and/or disc wobble would still see the pad(s) being forced continually outward raising the pressure momentarily and potentially lifting the pressure above the 2 psi set point leaving the pad in their excessive outward position again and we're back to where we started more volume being required to push the pads back into place.

There is now a possibility to install a 4 psi valve, this extra pressure may help in some cases of run-out/ in-balance or sticky brake cylinders by helping to maintain a higher pressure trapped buffer, but in my opinion this extra pressure is going to lead to glazing of the pads and an increased wear rate (excluding motor sports where sharpness of the brakes is traded against wear). If the braking system is in good overall condition, the pads won't be pushed back and will typically ride fractionally clear of the surface which negates any need for a residual pressure valve being installed in the first place.

Cheers Dave

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