Sp250 Brake Restrictor Valve

[timmartin]

This query is only about the Restrictor Valve normally fitted to the upstream side of the five way union. I am not attempting to reopen the massive topic of Sp250 braking which has been discussed so fully in other threads.
The valve in question is shown in section and explained on page N9 of the Service Manual. Although called a "restrictor valve" in the manual, it is a simple residual pressure valve designed to keep a residual pressure on all brakes in order to reduce "lost pedal".

My question is what is the correct residual pressure as set by Daimler?
I am aware that Wilwood market brake residual pressure valves with values of 2, 4 and 10psi, although they describe different reasons for fitting them.

I have just dismantled mine and found the inside to appear to be in excellent, apparently original, working condition - very close to as-new condition. I was amazed to find that the residual pressure was just over 36psi - as I have driven it tens of thousands of miles without any apparent binding or overheating.
Unscrewing the body equivalent to replacing the "Copper washer" (Fig. N4, item C) with a 1/4" spacer, reduced the residual pressure to 4psi. Set like this there was zero pre-compression on the spring. I cannot believe that it was designed to be assembled like this.

This does sound very similar to Larry's experience as described in his recent thread; perhaps they really were originally like this?

[wally999999]

My understanding is that the valve is intended to keep a fixed, minimum volume of fluid between the valve and the calipers. So the pistons can go back so far but no further - thereby preventing "loss of pedal". According the N9 in the manual; "...the amout of fluid in that part of the hydraulic system remains constant...", referring to the caliper - restrictor valve portion of the braking system. Pressure pulses generated by driving forces on the wheels / calipers lift the valve assembly to close the piston on its inlet side; which prevents excess fluid being returned to the master cylinder reservoir.

[daimlersteve]

now theres another thing i didnt know i didnt know.
Will check the release pressure on a new one i have and report back . 36psi seems a lot
Regards Steve

[Ian Slade]

36psi is not high when you consider the surface area of two pistons being pushed back by the disc down a 3/16 diameter pipe the advantage must be at least 100-1, for normal road use this will not be a problem as there will be very little flex on the axle causing the pad ride out, once the pads clear the disc there is little or no effect on the pads. Cornering at high speed on a twisty B road or race circuit using the engine torque in lower gears is another set of parameters which may overcome the valve if it is not in good condition allowing more than normal fluid back to the master cylinder due to greater flexing of the axle/half shafts.

[timmartin]

Thank you for your replies. A few thoughts before I reply to each. Please say where you think I am wrong.

I feel that he "Restrictor valve" succeeds in creating an admirable amount of mystery and misunderstanding for a little piece of steel with just one "Inlet port", one "Outlet port", one "Valve Assembly" and one "Outer Spring". There are no crafty drillings or passages. The "Valve assembly" is just a small mushroom shaped piece of steel with a loose "Valve plate" (meaning a simple washer) held against the underside of the mushroom head by a soft spring.

The valve can do very little:
- No fluid can flow through the valve in either direction unless there is enough pressure difference between the ports to compress one or other of the two springs. So if a calliper bleed screw is loosened and the brake pedal is not depressed, the fluid in the master cylinder cannot just run out.
- If the pressure from the master cylinder is a little greater than in the callipers, the weak spring on the "Valve assembly" is compressed as the "Valve assembly" (but not the "Valve plate" which is already hard against the opening in the "Screwed end connector") moves towards the callipers creating a space between the underside of the mushroom head and the "Valve plate". Fluid flows from the master cylinder to the callipers.
- If the pressure from the callipers is "sufficiently greater" than to the master cylinder, the outer spring is compressed as the "Valve assembly" and "Valve plate" moves as a single unit towards the master cylinder, creating a space between the "Valve plate" and the"Screwed end connector". Fluid flows from the callipers to the master cylinder.
My original question was what constitutes "sufficiently greater". Should it really be 36psi?

When the brake pedal is depressed, the centre valve in the master cylinder seals the high pressure (output) section of the master cylinder from the reservoir. In this condition there is no path for flow from the callipers to the reservoir, whether the restrictor valve is in working order or not.
But when the brake pedal is not depressed - whether during normal driving or after releasing the brakes - the master cylinder piston retracts, being followed by fluid from the brakes, and possibly a little top up from the reservoir if necessary. But when there are pressure pulses from the braking system, whether from pad knock back, flexing or whatever, fluid can be pushed all the way back into the master cylinder and up into the reservoir. There is nowhere else for it to go. After that there is less fluid in the master cylinder and the rest of the braking system, so the pedal has to be depressed further to get the same effect as before. "Lost pedal". A second pump on the pedal puts the fluid back in.

Using a "Restrictor valve" to resist the flow of fluid using back pressure against all but the biggest pulses does look like a logical fix.

Andrew. I think that you, Daimler and I are all saying the same thing, except where Daimler say that "... as ... hydraulic pressure ... momentarily increases and so will close the restrictor valve."
Since flow from the callipers to the fluid reservoir can only occur when the pedal is not depressed, the "Valve assembly" must already be in its closed position, only opening for pressures from the callipers which are greater than the set pressure (the 36psi?). If they mean that pressure pulses generated will cause the "Valve assembly" to move fully to the master cylinder end, thereby closing the "Inlet port", then perhaps they mean partial closure. Even if the pulses can move the "Valve body" until it hits the end of its possible travel, it will not seal that port. This because the cup shaped end of the "Valve body" has three recesses extending 50% round its rim circumference; this would allow some fluid to pass instead of sealing. I have rechecked the "Valve assembly" and "Outer spring" and confirm that there is sufficient free movement in the spring to allow the "Valve body" to reach the end of its chamber and the opening to the "Inlet port".

Steve. Thank you. I will be interested to see what you find and what you think.

Ian. I think that you may well be ahead of me in the understanding of what is pushing the fluid back, causing pad knock back etc. Whichever is the dominant cause, it results in a real problem for some people. I am not sure that I understood you about the significance of the 36psi. If my schoolboy sums are correct, 36psi will give a force on a front 54mm diameter piston and its pad of 127 lbs. For the back callipers my numbers are 43mm dia and 81 lbs. And then there is another pad pressing just as hard on the other side of each disc as well... What can the coefficient of friction be between the pad and the disc? It must be tiny. Why is it still easy to turn the wheels?

[Ian Slade]

Tim, I first came across pad rideout not long after I bought my first car, I was invited to join the DLOC team entry at Prescott Hill Climb, being new to competition I asked Duncan Saunders ( team leader) what speeds I should do from the start through the first bend, I was Informed as fast as you can go, this I understood to be it was OK to use the car flat out, wrong it was advice as to my capabilitiy, so flat out I went, approaching the hairpin I relied on a friends advice, don't brake until you know you cant make the corner then hit the brakes hard. Having followed this advice I hit the brakes and the pedal hit the floor and no decrease in speed, I did make the corner albeit doing a wall of death on the banking, that was caused by pad rideout due to the axle flexing through the first bend under full acceleration in second gear pulling 6.5k rpm. I mentioned the problem to Duncan to which he said pad rideout! Stripped the restrictor down cleaned it with Inhibasol rebuilt it and bled the system twice, this improved the rideout problem, but on the exit of all the corners I left foot braked gently to ensure there was a full pedal. I never used that car in competition again, but replaced the axle a year later due to the tubes becoming loose in the diff casing. I built another car for competition but with a larger servo and stripped the restrictor valve cleaned and rebuilt, this car was a A spec converted to a B chassis and some 2cwt less in weight to my C spec, however this car never had a rideout problem as serious as the C spec but when competing I always pumped the brakes on the straight after a corner, just in case