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Dart Brake Calipers

Vortex O'Plinth
Extra-Wise Man
Extra-Wise Man
Posts: 1065
Joined: Fri Feb 12, 2016 8:15 pm
Location: Gloucestershire

Re: Dart Brake Calipers

Post by Vortex O'Plinth »

daimlernorm wrote: Tue Apr 24, 2018 4:03 pm Interesting comment Nick about double pump on brake I have to do the exact same when braking on my SP
could you advise the Lucas Part No. for the Brake Lamp Switch.
The 'double pump' brake pedal action is experienced by many SP owners Norm. If you search the forum you'll find several sensible suggestions that owners have found cure the problem - replacing perished brake hoses and tightening up the front wheel bearings being two examples. As I stated, none of these logical solutions had any effect on my problem.

I can offer no guarantee that replacing the switch on your car will have a similar result, but for what it's worth the original Lucas switch part Number is 3980003. There are modern replacements for the Lucas original available from several sources - this one for example. I actually ordered from this site but instead of the switch illustrated (in the green box) I actually received a Lucas original - like this one.

The Lucas original has a larger body than the modern replacements, although the thread is the same. Whether this has any relevance to the problem I have no idea.
Nick

"Don't bother with the Air & Space Museum - there's nothing to see.......".

User avatar
Kbeal
Posts: 148
Joined: Mon Feb 15, 2016 9:01 pm

Re: Dart Brake Calipers

Post by Kbeal »

Double pump can be eliminated by fitting a new non return valve in the brake line as close to the master cylinder as possible.

Series 1 jag XJ6 3 pot calipers are a straight swap for the front SP calipers

Kevin

Paulkennedy
Posts: 135
Joined: Fri Feb 12, 2016 11:01 pm

Re: Dart Brake Calipers

Post by Paulkennedy »

Kevin when you say use a new non return valve is that something different to the original restrictor valve.
Thanks
Paul

timmartin
Posts: 65
Joined: Wed Feb 17, 2016 1:37 pm
Location: Berkshire

Re: Dart Brake Calipers

Post by timmartin »

I have been surprised by how seldom the original Restrictor Valve is discussed during debate about “loss of pedal”.

Reading again about the Resrictor Valve in the Service Manual, it appears to function as a “residual pressure valve”, so that after a brake application a small pressure is retained on each caliper to encourage any brake fluid that might otherwise be pumped back into the master cylinder - by piston knock back (out of true disc or wobbly wheel bearings) or flexing brake hoses - to stay within the calipers.

This appears to me to be the same principle as the Residual Pressure Valves available from Wilwood (2psi for disc brakes, or 10psi for drums).

Since there appears to be relatively little discussion on the forum about the Restrictor Vave originally fitted, I have found myself wondering how many people have checked that theirs are still working, or even still fitted. I have one, but I have not checked it.


Returning to the query about alternative calipers, I wonder whether anyone on the Forum can comment upon experience after changing to the Jaguar three pot front calipers?
The piston diameters are 57mm for the big single piston, and 40mm for each of the two smaller ones.
Can anyone tell me the diameter of the pistons in the standard SP250 front calipers?
Tim
Blue 1964 SP250 in Berkshire

User avatar
Kbeal
Posts: 148
Joined: Mon Feb 15, 2016 9:01 pm

Re: Dart Brake Calipers

Post by Kbeal »

Barry Thorne used to use the Jag 3 pot calipers when racing. He said it improved the braking no end and provided a better front / rear balance. If you stamp on the brakes with the factory set up it's the rears which lock up first - not ideal.

I have a set sitting in the shelf which I will get round to fitting one day.....

Kevin

andrewatcourtfarm
Posts: 24
Joined: Sat Jun 24, 2017 10:40 am
Location: Gloucester, Gloucestershire.

Re: Dart Brake Calipers

Post by andrewatcourtfarm »

It is probably too late by now but Fosseway Performance may be able to help you out.

The owner (I have forgotten his name) came to have a look at my MK2 SP250 brake calipers so that he could manufacture them for a customer. He spent a considerable amount of time taking measurements etc.

Contact details : enquiries@fossewayperformance.co.uk, 01386426249

They are bases in Blockley Gloucestershire GL56 9ER

Andrew Hope

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