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MG MGB GT V8 Factory Originals Technical - Residual Pressure Valves
|Has anyone had any experience of using residual pressure valves in the braking system and what are the pros and cons.|
|I assume that you are refering to a valve that maintains a couple of psi in the system to keep the pads and shoes close to the discs and drums.|
If the brakes where designed to need them they would be factory fitted.
If you have excessive brake pedal travel then check for :-
1) air in hydraulics
2) loose or warped discs/drums
3) worn or badly adjusted wheel bearings
4) worn pedal and pushrod
|Residual pressure valves must be used on a disk braked car if the master cylinder is lower than the brake cylinders and are sometimes used on race cars to keep the pucks from backing off due to vibration. I have to have one on my TD with B front suspension, but wouldn't use one on a straight MGB.|
HTH, George B.
|I am about to fit a proportonal valve (wilwood brand $39.90) into the brake lines of my modified MGA. The reason being the car is fitted with dual master cylinders (independant front and rear system) and as it is now the rears lock up way before the front discs come on line. Wilwood advised their dial proportional valve will allow me to adjust the rear line pressures to enable the discs to cut in first. Plan to install in next month so will advise how the valve works after testing.|
The car is set up with 4 Pot Wilwoods on front and SDI Drums on rear but has excessive pedal travel dispite a number of attempts at bleeding and turning servo round as set out in Roger Williams book. However the braking system was almost perfect before changing servo and master cylinder except the brakes locked on probably due to Silicon fluid being used in the past now DOT4.
I understand that Wilwoods are prone to this problem but is it a case of bleed and bleed again or try residual pressure valves.
Any help appreciated
|Your problem could be the volume of fluid the master cylinder can deliver versus what the wheel cylinders require. That this wasn't evident before the master cylinder was changed could be due to the locked up brakes. You will almost certainly require the proportional valve that Mark mentioned because of the SD1 bakes in the rear, but I don't think a residual pressure valve for the fronts would help much. Try adjusting the rear brakes to lock-up and test the pedal movement for reduced travel to help isolate the problem.|
HTH, George B.
I've got SD1 Vitesse calipers ( 4 pot ) which have 1 1/2 inch pistons and small rear wheel cylinders assisted by a standard MGB servo but I don't have excessive pedal travel when the system is properly bleed. However it can be really difficult to get the air out of the servo, 30 psi on the eezibleed or manual bleeding with the engine running and enough pedal effort to activate the servo has worked for me in the past.
I've heard rumours that wilwood calipers are prone to distortion but not other problems, I'll enquire amongst some friends and report back.
I aggree with George about checking the rears for correct adjustment and the possible need for a proportioning valve on the rear. I run one but only restrict the rears on the race track, for road use they get full pressure. Your SD1 rear cylinders will almost certainly be larger than the ones I use, but I don't have the dimensions to hand at the moment.
you should find that Vitesse calipers are 1 5/8" Dia-1 1/2 dia piston are are usually Princess 4 -pot.
You're right, I should really remember things like that. The same piston size is also used on range rovers which have used both two and four piston calipers at various times, so if you damage a seal while fitting it you don't have to buy another four pot set for just one item.
I hope to have more info to refute or otherwise my comment about the wilwood calipers after this weekend.
This thread was discussed between 08/09/2000 and 19/09/2000
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