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Anyone have any ideas on how to release the pressure differential valve on a master cylinder, its well and truly stuck and I have tried compressed air, tapping it gently, tapping it hard, pulling it and eventually threatening it with banishment. Nothing works and I am begining to think it is seized solid in the barrel, I can only think of re-assembling the thing and using the hydraulic pressure to force it out.

Any help welcomed.

 

Dave

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I bought a new master cylinder after trying all the above for two weeks off & on........Hope its not an early model SD1 master cylinder as they are like hens teeth. They are currently going well in excess of £300.......if you can find one. I eventually located one at Brakes International.

 

www.brakesint.co.uk

 

They were extremely helpful, even taking the time to look into more recent alternatives for me. I use the early SD1 master as it fits on the Tr7 servo (requiring only a 3mm spacer to tweak the pushrod length) and is a larger bore to suit the displacement of the four pot calipers I use.

 

Cheers

Andy

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Guest trminator

Hi

 

If the "pressure differential valve" is the piston behind the nut at the end of the master cylinder then I too need to remove this.  Unfortunately heat didn't work for me.  Any other idea are welcome.  I'm going to try something tonight with it that I thought of but would still welcome any suggestions.

 

-Kyle

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No the PDV is seperate from the M/C, its the cylindrical gubbins near it mounted on the strut tower. It has a valve in it to limit rear brake line pressure to ensure the rears don't lock. As I understand it rebuilds aren't available only new ones.

 

Jim

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I met a man from Essex at the TRackday (who's name I may eventually remember!) who had a 4.6 supercharged TR7v8 with a rear disk brake conversion.  He was able to adjust front/rear brake bias.  Presumably he is using some sort of special/modified PDV to achieve this?

 

Malcolm.

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Couple of things, on the SD1 cylinder I use the valve behind the nut on the Master cylinder senses a lack of pressure and switches a warning light on the dash! The cylinder on the strut tower is a proportioning valve that has a secondary function of isolating a leaking line to allow pressure to build up in the other. (Front or rear) The two valves complement each other in function.

 

Malcolm, re your bias question

There are at least a couple of ways this can be done. One is a bias pedal box which allows the driver to adjust the proportion of pedal force that acts on the two brake master cylinders. This is very good but relatively complex as it requires a modified pedal box, two master cylinders, adjustment mechanism and replumbing round the masters. Also note you typically lose the use of the servo. Note this is how the works rally cars were done. Another way is to modify the Tr7 PDV. Note that the PDV is actually a pressure proportioning valve as opposed to a pressure limiter. It works by allowing the front & rear pressures to increase together until a preload set by a spring is reached. At this time the pressure to the rear starts to increase further in proportion to the front. A full explanation is contained in the workshop manual if i recall correctly. It can be seen then that varying the preload on the spring will affect how this valve behaves. Increasing the preload puts more pressure to the rear, reducing it allows less pressure. This adjustment can be achieved by modifying the spring retainer cap to allow adjustment on the constrained length of the spring. This adjustment needs to be fine as small changes have a big impact. The first plac i saw this done was on Dave Bulman's Tr7 V8 he did with CCC magazine. It is available on the net at

 

http://www.triumph-cars.com/ccctrt.htm

 

Makes interesting reading. See article 4 for the brakes bit. It is worth pointing out that using either method and adjusting the brakes without an understanding of what you are doing can be dangerous. it is all to easy to have too much bias to the rear hence causing it to lock up first & you to spin out........Also note that the amount of rear bias you want increases as the available grip reduces. (Due to reduced weight transfer) Hence a tarmac rally car has much less front bias than a gravel car.

 

Hope that all makes sense.

Andy

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Hi

 

If the "pressure differential valve" is the piston behind the nut at the end of the master cylinder then I too need to remove this.  Unfortunately heat didn't work for me.  Any other idea are welcome.  I'm going to try something tonight with it that I thought of but would still welcome any suggestions.

 

-Kyle

Kyle,

 

The PDV is the valve behind the brass nut on the end of the master cylinder as you rightly say, the valve on top of the strut is the pressure reducing valve which effectively splits the pressure between the front and back brakes (back brakes at lower pressure) and prevents a leak emptying the reservoir by using lack of back pressure to seal offf the circuit leaking.

The PDV is effectively a shuttle valve which is central in the barrel when the primary and secondary circuits in the master cylinder are equalised (at the same pressure) and therefore protects against master cylinder failure or failure of the pipes to the pressure reducing valve.

Any loss of pressure downstream of the pressure reducing valve will not register on the PDV because it isolates the circuit when it loses pressure.

Anyway back to the question you asked, I heated the master cylinder body (slowly) and pushed the PDV further into the bore,   I then put a small flat screwdriver into the PDV switch port and pushed the valve back out again using the part of the valve which operated the switch. Doing this a few times released the stiction on the O-ring and allowed to pull the valve free with a pair of (very) long nosed pliers.

 

Sorry about the long winded reply but I am an engineer and can't help long replies!!

 

Dave  :;):

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Oops, noticed i have the bias the wrong way round on the last sentence above, more front bias on a tarmac car, more rear bias on a gravel car.......apologies for my typing not keeping up with my brain.

Andy

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