Looking for some guidance on selecting a master cylinder and how to plumb this in...

 

I'm fitting rear discs to my 1984 non-servo breadvan, using 226mm dia rear discs & larger 256mm dia front discs. Picking though threads in the forum indicates I need a larger master cylinder, most suggest going with something around 23mm bore.

 

Question 1: What m/c works best and/or is easiest to fit? Can the fluid reservoir be swapped from something else to make the install easier (hammering the bulkhead is last resort; making spacer to use servo'ed m/c s possible but I'd rather avoid the additional cost). Using a motorsport m/c with remote reservoir would achieve the desired goal, but picking something from a different car is bound to be cheaper...

 

Question 2: The current m/c appears to be only 3 ports - is this 1 line to each front brake, and single line to rears (presumably with T-piece to split fluid at the rear)? Most of the alternative m/c's appear to be 4-ports - how should these be plumbed in?

 

Question 3: The current m/c has a pressure switch mounted on the end, but the alternative 4-port m/c's do not seem to have this - what should I do with this switch? (Polite answers only ,please!)

 

Thanks,

Adam

id go with a golf mk2 m/c theyre about 23mm an cheap new

 

plumb it so the front pair of outlets do one front and opposite rear , and the rear outlets the other 2 corners ...so if something fails you have one font /rear brake  

 

the spacing for the res is common to most 80s/early 90s vag cars ...so yeah keep yours or find something else

 

id guess the switch might be the brake light switch?   if it is then later cars use one on the pedal so use that

 

will you be fitting the bias valve from a gt/g40...otherwise the rears will lock and spin you round in the wet

Thanks for that.

 

I had hoped that increasing the front disc diameter from 239mm to 256mm would help reduce the likelihood of locking the rears, but if a bias valve is still recommended that I'd better fit one (or buy some brown trousers).

If I go with the X-split front/rear plumbing, does that work with the bias valve? i.e. where in the lines does this need to go?

 

Cheers

golf gti came with 239 on early cars and 256 on later ones , they all had the bias valve ,in all honesty discs are too much for a light weight car on the rear , not needed , you often see the rears rusted up as the valve limits the use so much they dont work hard enough to clear the rust away

 

you would plumb both rear lines into the bias valve if using vw parts   

 

id put the money and effort into serious front ones , can always fit bigger drums at the rear but still not needed imo

I did wonder if the rear discs were overkill but got a bit carried away...

 

Is the VW valve to which you referred earlier one of these - this is load-sensing valve to reduce rear brake pressure in an unladen car, but presumably is not tune-able:

https://www.heritagepartscentre.com/uk/357612151-brake-pressure-regulator.html?gclid=EAIaIQobChMIypzl8qq07AIVB57VCh1obAY8EAYYASABEgKmJvD_BwE

 

So far searching for "Polo GT" or "G40" bias valve has not shown anything for sale in the usual places...

yeah thats it...2 pipes in , 2 out

Lovely - thanks!

So does this need to be connected to the rear axle to work as a load sensing valve, or just plumbed in to the rear lines to reduce the pressure to the rears? Assuming if the lever is not actuated by axle movement then if will provide the minimum pressure to the rears?

Edited 14 October, 20205 yr by Adam68
Additional question

yeah it needs to work as a load sensing valve...heres a quick sketch

 

 

 

 

I've had a poke underneath my car, and can confirm that it does not have a load sensing valve (I think some Mk2's did?)

 

So any hints. tips or sketches for the brackets & levers to mount the valve and actuate it would be much appreciated. The VW Heritage site has some photos of the valve installed (on a Golf?) so I have some idea of how to fit it, but mere details and dimensions would be great. Has this already been covered on the forum or in a thread?

 

Thanks,

Adam

no i dont have any dimensions , assume its much the same as the golf fitment ...seems alot of trouble to just have discs then dumb down the performance so they dont kill you 

Have a read through this https://www.clubpolo.co.uk/topic/265527-definitive-mk23-rear-disc-conversion-guide/ 

 

I've got a balance valve as standard with drums on the back. I think because the breadvan can take such a load over the rear axle.

 

I've got VWII calipers and 239mm vented upgrade at front.

Been a long time since I locked up the back wheels first but I did it once after the suspension was lowered.

(emergency stop for a cop car coming the other way in a hurry). Amazingly I stayed on the road on my side. 

There was a black double S on the tarmac for a few days.

 

 

The spring from valve to axle needs to be lightly tensioned with car and fuel tank empty.

 

Then put it on brake rollers and adjust. I think mine's about 2/3rds front 1/3rd rear biased with half a tank of fuel and a mechanic

in the driver's seat.

 

I'm fairly sure we only bent the spring to adjust for the 40mm rear drop.