TheSamba.com :: View topic - what master cylinder for 2 wheel rear disk, no front brake?

Mon May 03, 2010 10:46 am

hope all is well.
i searched and could find no info online or on this forum.
We have a low budget rail which only has rear brakes. the rail originally came with drum rear brakes, but we recenly installed porsche rear disks
from pick and pull.

the best way i can describe is the brakes are either on or off. it is too easy to lock up the brakes. even on a dry road, the brake lock up with very little pressure. hard to modulate.

any recommendations for a master cylinder for a car with only 2 brakes (rear disk)? this is one a lightweight two seater sandrail.

any advice or recommendations are greatly appreciated.

Mon May 03, 2010 11:03 am

You have too much brakes for the weight of the car.
You probably would have been better to go with Type 3 rear drum brakes.

Mon May 03, 2010 12:48 pm

I thought about posting this, but I am a racer (I ran an SCCA ITA, ITB, and FP car for years),and a newbe in this forum...so I don't know how all this will be taken but here she goes...

There is a lot that can come into play here, including pedal over all length, the length between the pivot point and end of the pedal. Then you have to look into the number of calipers per master cyl. and the number and size of each piston in each caliper and a few other things...a lot goes into building a brake system....and that is what you are doing.

...in short it is doubtful that you can pull calipers off a car, that I bet you picked because it was a Porsche and expect it to work right out of the box. I would also say that brakes are one of the few things I would be very nervous about using junk yard parts.

Sorry if I sound nasty but brakes are one area that you should not skimp on no matter what the use for the car is.....all that said....

Check out wilwood's site, you will have to have some specs on your system...stuff I talked about above, MS volume, piston size, caliper piston count and size, weight of the car is nice....look in the track car side of the web site and you should be able to find out what you need to do to get the car working like it should.

But please make sure the brakes are in good shape...hate to see anyone blow a seal and endup crashing into another buggy.

Mon May 03, 2010 1:30 pm

thanks for the honest advice.
we pulled the brakes after finding them on the junk yard with the generalization that disc brakes are generally better than drum brakes.

this is a group owned vacation project that sees the sand maybe twice a year. it only gets driven on soft sand dunes (pismo).

i was just wondering if another vw caliper would work with the disc brakes. I'm guessing that we need brakes with larger piston in the master cylinder
and was hoping some vw fanatic could recommend a model to choose from.

Mon May 03, 2010 3:09 pm

What master cylinder are you running?

Mon May 03, 2010 3:15 pm

its one of these
dune buggy master cylinders from jc whitney ...
doesnt say what size it is...

http://www.jcwhitney.com/replacement-dune-buggy-br...ilTabPanel

Mon May 03, 2010 3:31 pm

I think that is the same one I just got from Dan's. For a bus... I think it's 21 or 22mm diameter. By the way $30 from Dan's or Wolfsburg west.

The one I had for a 63 bug was 15mm or 17mm and will bolt in the same holes.

That's about all I can add to this. Let the experts help.

Mon May 03, 2010 6:20 pm

If you are running a 5/8 bore M/C, try running a 3/4 bore unit, larger surface area should equal slightly less braking pressure at the caliper. You can also extend the pivot distance so that the pedal stroke and length of plunger movement is increased, but you will still end up with a brake pedal that doesn't take a lot of pressure, just a greater amount of stroke distance before lock up.

Mon May 03, 2010 9:06 pm

Your master cylinder 22mm or i believe is 7/8, early bus . VW master cylinders have built in 10 lb. residual valves for drum brakes. se if you can remove this valve and replace with 4 lb.