Matching the correct master cylinder to disc brakes

[Jonny Grigg]

Just when I think I think i have this car almost finished- another little challenge to figure out.

So, my brake system uses Strange 2-pot calipers up front and 2-pot Jamar calipers at the rear. I did have a stage brake in the rear system but have now taken this out. The system uses a Neal pedal set up with an 'Afco' single circuit master cylinder. I have plumbed this using 3/16" copper pipe and have a residual pressure valve at the discharge from the master cylinder. from the RPV I have tee'd the line for the front and rear circuits. 

We bled the system and are pretty confident there is no air or leaks there now, however the pedal goes pretty much straight to the bulkhead. If you pump the pedal 2 or 3 times it comes up briefly (and a bit spongy), then goes 'soft' again....... when we had the stage brake in the rear circuit, by a combination of pumping the pedal and the stage brake we managed to get the rear brakes on hard and locked the stage brake out- then the fronts had a lovely pedal and worked great. 

I am guessing that the M/C is undersized for the car. it was the one that came with the set-up, but without draining it down and removing it from the car I can't be sure for certain what size the bore is- It may 5/8", but more likely 3/4" (i would hope...). One good recommendation I have been given is to adapt the system to use a VW dual circuit M/C, but this is quite a lot of fiddling about to make it fit (my car uses the 2.25" vertical flange pattern). The VW M/C is 3/4" bore I think, so I could use a Wilwood equivalent dual circuit, or perhaps a 1" bore single circuit, which looks a direct replacement for the Afco cylinder. The car also has chute so dual redundancy is not a major issue for me on the brakes.

Can anyone help me? Is there something I have done wrong with the original component selection- if so what would you recommend I change? What M/Cs are used with 4-wheel disc brakes on race cars using Strange (or equivalent) calipers?

thanks for the advice.

Jonny

[richie]

Jonny,

I had alot of problems with my strange calipers,the pistons just wouldn't stay out,they kept returning back to zero,which meant there was always alot of pedal travel to take up the adjustment,I had to narrow the pads to get the piston to move from the starting position,once away from that point they were fine,but took alot of head scratching to find the problem.

If you pump the system and hold your foot on the pedal does it stay firm? its possible you have a real small leak somewhere still

I have a wilwood master cylinder with 4 wheel strange calipers before[now 4 pot wilwood calipers on rear],cant remember if its 3/4 or 1inch but if everything is bled ok and the pads are pushed out onto the discs the pedal shouldnt move that far even with 5/8 master cylinder,the fluid movement would be small still

cheers Richie

[JamieL]

Jonny,

I had alot of problems with my strange calipers,the pistons just wouldn't stay out,they kept returning back to zero,which meant there was always alot of pedal travel to take up the adjustment,I had to narrow the pads to get the piston to move from the starting position,once away from that point they were fine,but took alot of head scratching to find the problem.

If you pump the system and hold your foot on the pedal does it stay firm?

cheers Richie

Hi Richie,

We have now tried 3 different configurations with these brakes....

1. Rear brakes only with hydraulic handbrake.
2. Front and rear and handbrake
3. Front and rear, no handbrake

Each time, the system bleeds nicely and works with pumping. Once pumped, pedal stays firm and braking effort maintained. However, all pedal feeling is immediately lost as soon as the pedal is released, until pumped up again.

To me this means the MC is too small diameter/bore and not pushing sufficient volume per stroke to operate caliper pistons sufficiently.

That said, I/we also witnessed the exact same with rear calipers not staying out - as if the return spring is too strong and residual pressure not maintained - hence fitting the RPV's, but to no effect...

The other strange thing that I haven't seen before was that, when pumping to build pressure, fluid was surging "back" into the MC reservoir...

Personally I think retro-fitting a VW MC is the wrong way to go (too many adapters and subsequent bad packaging on car) and think a correctly sized (7/8" or 1") Wilwood or similar MC is preferable.

Slightly ironic that these brakes are stopping us getting going at all - I'll try to resist the urge to mention the 3 different size narrowed, powdercoated front beams and steering linkages... 

[Jonny Grigg]

Richie,

Is your set up single or dual circuit? Which M/C do you use?

Thanks

Jonny.

[Udo]

You need a M/C with big diameter pistons like 25 mm . Willwood single or i try a bus T2  M/C with 2 pistons on my wifes car now

Udo

[DWL_Puavo]

Supposedly you're not using residual pressure valves in the master cylinder? We fitted our track car with those to ensure exactly the same pedal feel in every braking. I've heard that caliper pistons can pull back in because of vibration and even with the smallest of disc flex during hard cornering. Late bus m/c is also aluminium

[Jonny Grigg]

Thanks the help. I ordered this today:

http://www.wilwood.com/MasterCylinders/MasterCylinderProd.aspx?itemno=260-6765

Arriving tomorrow so hopefully we can get it on the car tomorrow evening. Lets hope this works 

[Jon]

Even the stock VW master cylinders have a kind of residual pressure valve. The pads are supposed to only release pressure on the discs, not move away from the rotors any real distance. As I have understood it.

[JamieL]

Success! 

[Jonny Grigg]

Yes, we have a good brake pedal now 

Let's get this car finished....... 

[richie]

Excellent

was it just the master cylinder bore size or something else?

When you planning on going to the track?


cheers Richie