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SP250 residual brake pressure valve and remote servo - brakes binding?

Agnello11
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Joined: Tue May 21, 2019 4:07 pm
Location: Thames Valley

SP250 residual brake pressure valve and remote servo - brakes binding?

Post by Agnello11 » Fri Aug 09, 2019 10:07 am

It has become clear that I have an issue with the brakes on our SP250 as they are binding....
Initially it was concluded that the Lockheed remote servo was to blame as the brakes were slow to release after letting off the pedal. This seems to be a pretty common issue widely reported in Scimitar circles with some simple fixes, which were duly undertaken. That fixed the issue and the brakes responded properly and didn't bind at all.... for a while.... then binding and slow release symptoms returned and the assumption was that the servo was actually caput....

A 'new servo required' was the diagnosis.

A new remote servo (as found on eBay) was acquired and fitted. It's now been bled through.... but the binding symptoms are still present, so clearly the servo isn't the root issue :evil:

Attention has now turned to the 'restrictor valve' (part labelled '46' on plate 21 in the parts diagrams) or residual pressure valve that is screwed into the top of the 5-way connector in the brake lines. It's designed to retain a little bit of pressure in the brake calipers to keep the pistons and pads lightly on the discs and prevent 'dead pedal', where you come to press the brake pedal and find it goes to the floor taking up the slack from where pads have been crept back into calipers as a result of flex between components. Now I don't know whether this is even a 'thing' on Darts and whether the restrictor valve is really necessary.... but I guess it was originally fitted for a reason.

A question that has come to mind is does a remote servo have a residual pressure valve or some residual pressure function? We are wondering if it's possible that we have inadvertently doubled-up on residual pressure valves having fitted a remote servo and that the binding issues we are experiencing are as a result of the original restrictor valve having too much back pressure because of the servo....

Has anyone else fitted a remote servo to a Dart? If so, what did you do with the 'restrictor valve'?

I see that the original restrictor valves are no longer available so it'll have to be either rebuilt or a modern alternative adapted to fit, but does anyone have a clue what sort of residual pressure it should be providing? I would assume about 2 psi, but anyone know for sure?

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John-B
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Re: SP250 residual brake pressure valve and remote servo - brakes binding?

Post by John-B » Fri Aug 09, 2019 10:19 am

I'm not answering your question directly, but there were several discussions on the old forum about the need for a restrictor valve. My car and many others don't have one, but I haven't got a servo.

Do a search http://archive.dloc.co.uk/forum/search.asp . Here's one of many discussions
http://archive.dloc.co.uk/forum/topic.a ... ctor,valve

Perhaps a cheap solution in the short term would be to remove the restrictor valve and see what happens. If you have a new servo, perhaps it isn't at fault.

There are stories of brakes locking after hard braking.

silverdart
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Location: West Midlands

Re: SP250 residual brake pressure valve and remote servo - brakes binding?

Post by silverdart » Fri Aug 09, 2019 10:31 am

Hi, I had exactly the same problem but, I do not have a servo fitted.
In my case it was a rear brake problem and the issue was solved by replacing the flexi hoses which were restricting the flow of brake fluid.

Dave.

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Jez
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Joined: Mon Feb 15, 2016 8:00 am

Re: SP250 residual brake pressure valve and remote servo - brakes binding?

Post by Jez » Fri Aug 09, 2019 10:51 am

My set up has a remote servo but no restricter valve. I do get the 'dead' pedal effect from time to time on first push but otherwise brakes are excellent for a '60's car. At some, undefined, point I will refurb the system, however, at which time I plan to fit a valve along with new flexi pipes, seals etc.

Cheers - Jez
Jez Stow
1960 Daimler Dart
and a few other toys

Agnello11
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Joined: Tue May 21, 2019 4:07 pm
Location: Thames Valley

Re: SP250 residual brake pressure valve and remote servo - brakes binding?

Post by Agnello11 » Fri Aug 09, 2019 11:06 am

Thanks, I'll have a search through the archives....

The restrictor valve is pretty much the only thing left on our braking system that hasn't been changed.... It's had new flexi hoses all round already, all calipers rebuilt, new master cylinder and now a new servo....

Interesting to know that a number are running with no restrictor valve without issue...

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John-B
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Re: SP250 residual brake pressure valve and remote servo - brakes binding?

Post by John-B » Fri Aug 09, 2019 11:26 am

Agnello11 wrote:
Fri Aug 09, 2019 11:06 am
Interesting to know that a number are running with no restrictor valve without issue...
Well, I usually double pump to get a nice firm feel, but the car will stop without double pumping.

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JohnM
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Location: N W London

Re: SP250 residual brake pressure valve and remote servo - brakes binding?

Post by JohnM » Fri Aug 09, 2019 4:41 pm

Ours has a remote servo and no restrictor valve - my "reading" of restrictor valves is they're really only needed for competition cars where hard cornering and vibration can cause pad knock-back.
You can easily dismantle the valve and leave out the innards in case there's some sort of blockage in it, and then you don't have to make up a new brake pipe.........

Beware of Lockheed-style servos that are currently available via Ebay etc if you don't trust the source - there are (literally!) Chinese copies out there that look exact on the outside but have poor fit inside, and a sticking operation piston would cause these symptoms - if the brakes are locking on, I might still be suspicious of the servo (I've had it happen on 2 different cars), but if they're just dragging I'd take the innards out of the restrictor valve.

Just my 2p worth, of course, but hope it might help.......
regards,
John
John M in Middlesex, NW London
1962 SP250 "B" Spec

Ian Hastings
Posts: 104
Joined: Tue Mar 15, 2016 3:00 pm

Re: SP250 residual brake pressure valve and remote servo - brakes binding?

Post by Ian Hastings » Sat Aug 10, 2019 12:46 am

One thing to check is the master cylinder. If there is slight corrosion or dirt just inside the cylinder under the rubber boot or if the pedal clearance is not adjusted correctly, then the piston cannot return back to its rest position thereby blocking the aperture between the cylinder and the fluid reservoir.
If this happens the fluid cannot return to the reservoir and the brakes will lock on or bind.

Ian Slade
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Location: Akrotiri Cyprus

Re: SP250 residual brake pressure valve and remote servo - brakes binding?

Post by Ian Slade » Sat Aug 10, 2019 6:14 am

Fully agree on the free play, however the restrictor valve is what it says, it restricts the flow to the rear brakes to avoid lock ups, it does not hold a pressure and shouldn't hold a pressure. To check where you think the there is pressure hold on the brakes, put the car on axle stands, check the brakes are binding by spinning the wheels, open a bleed nipple and check if the brake frees, if so the slacken the union to the top of the restrictor valve and check again, if this clears the problem remove and clean the valve and ensure it is assembled correctly, if the brakes do release when the union is slackened then your problem is with the servo, master cylinder or pipework, or you can just slacken the restrictor union and work backward or forwards depending on the result.
Owner since the 70's, Genghis is slightly to my left.

dovr
Posts: 3
Joined: Mon Feb 22, 2016 7:51 pm

Re: SP250 residual brake pressure valve and remote servo - brakes binding?

Post by dovr » Sat Aug 10, 2019 9:00 pm

You mentioned that you changing the master cylinder, might be worth checking the activation rod length. Ensure there is no tension /pressure between the rod and pedal

If set incorrectly it can cause binding.


Cheers

Dov

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