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SP250 Restrictor or non-return valve for brakes

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Location: Berkshire

Re: SP250 Restrictor or non-return valve for brakes

Post by timmartin » Wed Apr 25, 2018 5:53 pm

It had not occurred to me that 2psi might not be enough, although I had seen that Wilwood’s explanation for its purpose was rather different from the Daimler words that you quoted, Nick.

10psi does seem an awful lot. Probably enough to keep the discs nice and bright. Possibly enough to keep them nice and warm as well?
I shall be seeing Barry tomorrow, and will see what he says.

I suppose that it would make sense to try my own brakes without the Restrictor Valve as a comparison and to take my valve apart to check on its condition. Bit of a chore, especially as I have only just renewed the fluid and my number one mechanic assistant (daughter) is not often conveniently to hand.

A completely different, and much more expensive, approach is to use a larger bore master cylinder and a servo to bring the pedal pressure back to where it started for the same retardation. And an even more expensive if one is to use a tandem master cylinder with one of those nice Spanish double circuit servos.

As an example, wouldn’t doubling the master cylinder piston area and using a 2:1 servo boost result in half the lost pedal?
Is this why modern cars seem to favour servo assisted pedals with large diameter master cylinders - giving amazingly heavy brakes if the engine stops...?

Using a satisfactory restrictor/residual-pressure valve does have great attraction in terms of simplicity and cost. It will be interesting to hear people’s experiences.
Blue 1964 SP250 in Berkshire

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Re: SP250 Restrictor or non-return valve for brakes

Post by DaveM » Thu Apr 26, 2018 10:20 am

To answer your question regarding the RPV valve and whether to use it or not requires a bit of background before answering. I've spent considerable time calculating and considering the various options of the braking system during my restoration to try and achieve just the right balance and brake feel, especially as this was going to be the wife's car when finished.

What I finally settled on was keeping the standard rear callipers with twin 50mm pistons and installing Jaguar XJ6 Series 1 three pot callipers on the front that have 2 x 42mm and 1 x 57mm pistons which in turn give a 15% increase in front brake bias, 4 pot in my opinion would shift that bias too far forward. I also replaced all brake lines, installed braided hoses and greenstuff pads. On trying the brakes with the standard 3/4 master cylinder the pedal feel was terrible, 4 to 5" long and near triple push to harden up. I wasn't surprised by this as I'd increased the the overall system volume above the displacement of the master cylinder. I then considered the pedal ratio of the existing pedal box at 5:1, which is at the upper level for a powered brake system target of 4:1 to 5:1, so I went with a 3:1 servo and Girling 7/8" master, these two things combined have absolutely transformed the brakes beyond belief with no double pedal requirement at all. The total pedal travel has reduced to approx' 2.5" before hydraulic lock, a simple light touch is required for normal slowing, just over 1.5" gives a quick stop and a full emergency stop from 30mph requires about 2" with the fronts locking up and the backs skipping two or three times. More importantly the wife loves the brakes and feels total confident in them.

A couple of other modification were carried out, one being the introduction of a brake fluid level alarm (£12) as a potential issue with using a remote servo is that there is one seal between the brake fluid/vacuum chamber, a leak here could see your brake fluid evacuated into the intake manifold. The other was to remove the hydraulic brake light switch altogether and install an electric contact switch on the pedal box.
So back to the PRV, I've had it inline and out of the system several time during testing and can say I've felt no difference either way, on both old and new systems with regards to pedal travel/ pumping up the brakes, however this might be because everything on the car is brand new, with new bearing and zero disc wobble, so the function its was originally fitted for may not be evident in my car making it obsolete. I also question its worth at 2psi in a 1000psi+ system and its lack of use in modern systems, but for £20 couldn't see an issue with retaining it either so I did.

In my opinion after spending so much time on the brakes is that the master cylinder size is the culprit of the double pumping phenomena and by increasing from 3/4" to 7/8" gives an increase in pedal effort if no servo is fitted but displaces 20% more fluid so travel is reduced and hence double push. If required, the extra effort can be factored out with a corresponding servo.

Cheers Dave

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Re: SP250 Restrictor or non-return valve for brakes

Post by daimlersteve » Thu Apr 26, 2018 11:16 am

re more modern tandem master cylinders --- what looks like a steel adaptor at the outlet is actually a pair of residual line pressure valves. Steve.

Ian Slade
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Re: SP250 Restrictor or non-return valve for brakes

Post by Ian Slade » Thu Apr 26, 2018 11:58 am

It really depends on your driving and where you drive, it is highly unlikely if you live around the M25 and use it the double pump will not be required or the pad ride out will occur unless the rear axle tubes are moving. From the last time I drove down south it is also unlikely you will experience pad ride out as there's so much traffic you will be lucky to achieve ride out as your cornering speeds will be below the ride out speed needed to cause it. The car needs to be driven fairly hard on the road and using the power and torque of the engine, in these days you will come to the attention of that rare sight, the long arm of the law ( from what I have heard), the average owner won't experience it if the rpm is kept below 5000rpm, if it does you have another problem
Owner since the 70's, Genghis is slightly to my left.

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Re: SP250 Restrictor or non-return valve for brakes

Post by John-B » Thu Apr 26, 2018 1:09 pm

Reading between the lines above it seems that it doesn't make a lot of difference to the feel of the pedal whether you have a restrictor valve or not. I've always double-pumped to get a harder feel to the pedal with all my old cars. If I have to brake suddenly without double-pumping the brakes still perform well, but with a longer pedal travel.

I was also told that pumping several times on icy roads will help prevent the brakes locking, so it's a habit I have got into.

However, one comment on the old forum was that some people who have got a restrictor valve get brakes locking on which is dangerous because you skid to a halt and have to get under the car to release a bit of fluid to release the pressure, so that's one reason not to have a valve.

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Re: SP250 Restrictor or non-return valve for brakes

Post by timmartin » Fri Apr 27, 2018 12:15 pm

While collecting some parts from Barry yesterday, I took the opportunity to ask him what he thought about the valves.
He said that he did not know the residual pressure of the original Daimler units, but observerved that there must be a great many of the original ones which either no longer work correctly or are missing completely.
He keeps the 2psi Wilwood units in stock for those who wish to fit them.
Blue 1964 SP250 in Berkshire

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Re: SP250 Restrictor or non-return valve for brakes

Post by Fossil » Sun Apr 29, 2018 10:55 pm

In 1999/2000 I upgraded the brakes: I reconditioned the calipers with s/steel pistons, braided hoses and Green Stuff pads, plus changed to silicone fluid. I had fitted new discs, Kunifer pipes and a servo some years prior to that. Back on the road all 4 hubs were warm after a few miles, so my first recourse was to remove the restrictor valve. The contents were unrecognisable and surrounded by black goo. So I left out the contents, cleaned the valve body and replaced it in order not to have to make up a new pipe again. The hubs returned to their normal behaviour and the brakes remain excellent, with no need for a double pump.

I've tended to use what I think is called "cadence braking" for many years, ie frequent application of the brakes to slow the vehicle in steps; perhaps because I grew up in NE Scotland in the 50s/60s where snow & ice were normal for much of the winter. I still do this in anything I drive which is probably why I'm not aware of requiring a double pump in the SP.



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Re: SP250 Restrictor or non-return valve for brakes

Post by Warsash 2 » Mon Apr 30, 2018 11:31 am

I cited Dual circuit brakes to Margaret's SP which removed the valve but with the extra fluid a by product was the need for dual pump disappeared

David S
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Re: SP250 Restrictor or non-return valve for brakes

Post by David S » Mon Apr 30, 2018 2:01 pm

I agree with DaveM wholeheartedly!
On my SP, to avoid the long first push [double pump seems too serious!], I put on a 7/8 master cylinder and a smallish 1.9:1 servo.
Brakes feel like original but the pedal push length is now consistent.
I can see a 3:1 servo would be better with the Jaguar XJ6 calipers but may be too much for a standard car.

And btw, the wheel bearing movement is what must cause the inconstancy in pedal travel, but if you tighten bearings too tight they will fail early.


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Re: SP250 Restrictor or non-return valve for brakes

Post by Reedweaver » Sun Jun 02, 2019 8:25 am

Wilwood have now released a new 4 PSI-rated Residual Pressure Valve (RPV) to their line of pressure and flow controls. The 4 PSI Gold valve adds an option between the existing 2 PSI Blue and 10 PSI Red valves on disc brake systems where extreme shock loads, hard vibration, and suspension component flex can contribute to excessive caliper piston retraction and pad knock back.

The 4 PSI Gold valve uses the same 1/8”-27 NPT female thread in the aluminum body as the 2 PSI and 10 PSI models.

The only place in the UK where I have found them is here: ... s_id=25151

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