Good afternoon!

Hopefully you guys can help...

I've got an OBD pedal box, with twin master cylinders. One 0.625 and one 0.7. I have Brembo 4 pot calipers on the front (mk1 focus RS) and standard sierra calipers on the rear.
I bought the pedal box and master cylinders new about 4 years ago from OBD.

All the calipers are known good ones, all the lines are new stainless braided ones, there are no leaks on the system.

I've bled the brakes and there's no air coming out of any of the nipples but I still have a very spongy pedal, to the point the pedal hits the bulkhead with relative ease.

I'm thinking, incorrectly sized master cylinder(s) or knackered master cylinder(s)...

If I have to replace the master cylinders what sizes would be recommended?

Thanks,
Andy

how have you mounted the rear callipers?
if they are upside-down you have to remove them to bleed them

As already pointed out the bleed nipples need to be at the top so if they aren't you need to unbolt the rear callipers and place a packing piece to stop the piston from traveling too far before bleeding . In the past I have had to manually adjust the rear Pistons to get a decent pedal, also do not operate the hand brake before bleeding as this can cause problems. How are you bleeding the brakes?

Thanks for taking the time to reply.

All the calipers have the nipples to the top.

I probably could have explained a little more... I used to have a Sierra pedal box and master cylinder fitted on exactly the same setup, and I never had any issues like this.

The only thing that has changed is the pedal box and master cylinders.

Thanks,
Andy

Relatively easy to test the master cylinders by disconnecting the outlet pipe , fitting a bleed nipple and bleeding,. You should then have a solid pedal, if not one or more of the cylinders is faulty. You could if needed test each calliper in much the same way.

How does the volume of the Focus calipers compare with std Sierra ones? If it's significantly bigger then you'll need to increase the size of that master cylinder to match. Sounds like you have the same cylinders as me but I've currently got std Sierra fronts with no issues and a nice pedal.

As Rusty Nuts has said, how are you bleeding the system?

Twin master cylinders with a balance bar activate both front and rear brakes at the same time, so trying to bleed one corner at a time simply transfers part of the pedal movement to the other end of the car from the one you're trying to bleed.

Tilton recommends that you bleed both ends of the car at the same time: Link (see 2:12 onwards).