Master cylinder questions and hydraulics assisted brakes conversion

[Skull]

Hello.

After the small reds 314mm conversion i have been thinking about brakes...

1) According to the family album, in my compi i have the master cylinder with the sizes: 23,81x13x22,5 (Girling) (smaller than the s2/rs2 piece), but im not sure because i cant find the part number on it.

I have at home a master cylinder from an 1992 urs4.

the family album shows it should be 25,40x13x22,5 (same dimensions as the s2 aby MC)

the part number of the urs4 MC is something like 154 135 617

Now the obvious questions:

Will the urs4 MC fit my brake servo "drum" (i think it should)

will there be any gain in brake power?


2) I also have the hydraulic brake servo from the urs4. according to the PN should be the same as the s2 piece.

What else do i need to install it?

Would the tandem pump from an s2 fit on my engine?

Is the bomb a must?

Enough for now

Thanx!

[Skull]

strange part number

[johne]

If you use a hydraulic brake servo rather than a vacumm one you'll need a bomb for two reasons:

It's the bomb that regulates the servo pressure, not the pump.

Without the bomb you've got no pressure reserve to assist the brakes if you brake more than once in quick succession

John

[Error404]

IMHO Rok the vacuum servo is a better option. You will need to fit the S2 tandem PAS pump for a start, plus the bomb as johne said - more crap in teh enigne bay and more expensive stuff to go wrong (hydraulic servo is a PITA to change springs to mind )

Your master cylinder should be fine with the brembos, plenty people with stock S2 cylinder running those huge 8 pots without any problems, i'm not sure you need to move any extra fluid mate

[johne]

Your master cylinder should be fine with the brembos, plenty people with stock S2 cylinder running those huge 8 pots without any problems, i'm not sure you need to move any extra fluid mate

and I read a post stating that the 4 pistons in the small reds have almost the same surface area as the two pistons in the standard caliper which is why the standard S2 master cylinder will do just fine. Certainly does on my car with 996 Brembo calipers.

John

[Skull]

thanx gusy, looks like i will leave the vacuum drum on.

my master cylinder is smaller than the S2 unit. i will try to fit the urs4 piece today, looks like a pefrect fit.

my MC was designed for two 54mm pistons, now i have 4x40 and 4x36mm pistons. so there should be a difference?

[johne]

4 x 40mm pistons is 5026 square mm
4 x 36mm pistons is 4071 square mm

which gives a total for the Brembo caliper of 9100 square mm.

2 x 54 mm pistons is 4580 square mm - but due to the sliding caliper arrangement the effective area is twice this i.e. 9160 square mm

Hence the two calipers are almost identical in piston surface area and thus identical in pedal travel for the same master cylinder bore.

John

[Skull]

johne: so basically my stock MC should be ok...

i fitted the urs4 MC today, bleed the system and...

the brakes work, but, the pedal feels very hard, i cant lock the wheels on the wet.

will try again tomorrow.

[Skull]

another thing:

when i turn off the engine, and have the brake pedal pressed in, the pedal returns up to the top, even pressing my foot up?!?

[johne]

Is the non return valve in the line between your servo and the inlet manifold not working, not fitted or fitted the wrong way round?

John

[Skull]

i did not touch any vacuums...

what is the nipple on the brakefluid oil tank for?

I have a hose attached to it, but i dont know where it goes...

[johne]

It goes to the clutch master cylinder - it's the fluid supply to it since the brakes and clutch share the same fluid reservoir.

It's set up from the bottom so even if you loose all the fluid out through the clutch system there's still some left to work the brakes.

John

[Skull]

ok

is the master cylinder a 2 stage thing?

because looks like the first half of the travel work as it should, but passing the half was it is hard as a rock...

[Skull]

i have replaced the s4 MC (probably busted) back with the stock piece, and bleed the think properly, and now it works as it should.

but the nose dive is calling for a rear brake upgrade

[A80Avant]

i may need to replace the master cylinder on my rs2(if chaning the brake fluid does not save my A#$#*...

part number: 4A0 611 021 C

anyone knows where i can find a good one before biting the bullet and buying a new one from the stealers?