Ever since I fitted my 4 pot calipers when I press the brakes firmly my OSR caliper locks up almost straight away,it never did this when the XR2 calipers were fitted. I have the restricting valves that screw into the master cylinder to reduce rear brake pressure and the NSR one is fine just the drivers side is too sharp.
Does anyone have any ideas why this is happening now or how to go about curing it. Like I say it was fine on the XR calipers and has only started this since fitting the 4 pots. It's the Sierra Cosworth rear calipers. Cheers

Restrictor valves plugged in the right ports?

possibly corner weighting out, and the extra braking of the fancy 4 pots is showing it up??

It's a Mondeo master cylinder I'm using and the restrictor valves are different thread sizes so you can't really go wrong. I thought it might be because I now have Ferodo DS250 pads up front and cheap £12 ones on the rear. It is odd how it was fine with XR2 calipers on the front but now I've fitted the Wilwood 4 pots the OSR is locking up way too easily.

[Edited on 12/2/15 by Scuzzle]

have you worked out the difference in piston area from the xr2 to wilwoods? could just be that you need to change your Rr mastercylinder size assuming 2 m/cyls for the braking?

summat like this (2sec google) may help work out if its piston size issue
cheers
CNH

[Edited on 12/2/15 by CNHSS1]

I worked it out and it should be OK, I'm using the 31.8mm, I just can't figure out why it's the OSR only, the NSR is fine. When I ran the XR calipers I could jump on the pedal and it would stop evenly now the OSR lock up far too easily every time.

The OSR isn't seized is it?

Corner weights are the first port of call. The stiffer the car is sprung the more critical detting the corner weights becomes. However if you stop the offside rear loving it increase the tendency for the near side rear to lock.

The OSR is not seized, it turns freely however it's always that side I've had issues with. I used to run adjustable in-line bias adjusters but I gave up on them as they were always leaking or holding pressure, when I binned them and went to the pressure valves in the master cylinder things got much better.

if any were seized or binding it is the nsr to check obviously the osr is working fine.
also could be with the increased braking efficiency on the front it is lightening the rear almost like bunny hopping allowing the rear to lock abruptly as the load lightens under braking. In your post you said when "pressed firmly"

Check the nsr for any piston binding seizure, check statically with rear jacked up , pedal pressed osr locked is the nsr?. once the rear axle imbalance is sorted then work on the front rear balance

had a similar problem when we messed about with mini's and the problem was the one not locking
Good luck gary

[Edited on 12/2/15 by garyt]

Maybe one of the pressure limiting valves isn't sealing correctly. If you swap the valves and the problem moves to the other side then you've found the problem, though a bit of a pain to have to re-bleed.

The issue will be down to weight transfer under braking. I'm still using XR brakes on my Zetec powered Mk1 Fiesta hillclimber and using sticky Avon slicks. I have a couple of pictures of the rear wheels off the ground under heavy braking. With mine having no interior, perspex windows and glass fibre tailgate the weight distribution is 80/20 front to rear.

Thanks guys, I think I will bin the Mondeo limiting valves and invest in a decent adjustable valve setup, I had some previously and one leaked from new and the other held pressure on but they were cheaper ones so I suppose it's a case of you get what you pay for.