OBP Pedal Box in a Classic Mini... Problems. Please Help!

[chris_rd92]

Hi everyone!

Fist off, sorry if this is in the wrong section, I'm new here so wasn't sure where to post and took an educated guess. Feel free to move!

So as a sort of introduction, my name is Chris and I drive a highly modified Classic Mini which I've restored twice in 9 years. My latest modification is the addition of a brand new OBP Motorsport full pedalbox conversion (this one - http://www.obpltd.com/Pedal-Boxes/Classic-Mini), and I am now having all sorts of problems with it. I wasn't sure where else to turn for advice so figured this might be a good place on the off chance that anyone here has experience with bias / dual mater cylinder pedal boxes.

I'll go into detail now regarding the problems I have. I have a .625 cylinder on the front, and a .75 on the rear, with completely standard Classic Mini calipers, disks, and pads. The pads and disks are not new, but still have a lot of life left on them.

Yesterday I managed to get the whole system 100% air free and the pedal felt rather good. This morning however I went out to the garage to discover the pedal had become very spongy overnight, going almost to the floor. The first thing I thought was I had a leak, so I searched the entire system repeatedly and found everything to be perfect, no leaks at all. The reservoir is also topped up full and there are no bubbles in the reservoir when the pedal is depressed. There is absolutely no air in the system so I am stumped.

The next problem I have is that the front brakes are binding incredibly badly even when the pedal is not being pressed at all, to the point where it takes a lot of effort just to make the wheel turn by hand. The rear brakes are not binding at all. I just don't understand how the brakes can be almost solid while the pedal is so soft.

I have the cylinder push rods adjusted so the front is slightly longer, and the bias bar was originally set up on the workbench so that the bias bar would straighten out as the pedal was pushed, before applying slightly more pressure to the front than the back. However, now while looking at the bias bar while depressing the pedal, it appears that it will initially straighten out, but as the pedal is pressed further it shifts most of the movement back to the rear cylinder again. No amount of adjustment on bias bar seems to solve this.

I'm pulling my hair out over this, so any help regarding these problems would be much appreciated!

Cheers
Chris

[interloper]

First thought has one of the master cylinders got a duff internal seal (so the fluid is pushing past the cylinder). No idea why it's pressuring the front calipers - assuming you haven't got a seized caliper?

I guess I would want to check over everything other than the pedal box first, check your calipers, make sure you haven't got a kinked brake line, check the hoses etc, etc..

[Skyedriver]

Might be better off in the Classic Mini forum (see below) or maybe even the Kit Car forum if it's heavily modded

[chris_rd92]

interloper said:

First thought has one of the master cylinders got a duff internal seal (so the fluid is pushing past the cylinder). No idea why it's pressuring the front calipers - assuming you haven't got a seized caliper?

I guess I would want to check over everything other than the pedal box first, check your calipers, make sure you haven't got a kinked brake line, check the hoses etc, etc..

Calipers seem to be fine but I think I'll rebuild them anyway just to cross that off the list. The master cylinder is brand new from OBP so I would be surprised if it was knackered already, but it is a possibility! I've been over the whole system like a hawk and all lines and connections seem perfect.

[chris_rd92]

Skyedriver said:

Might be better off in the Classic Mini forum (see below) or maybe even the Kit Car forum if it's heavily modded

I considered posting in the Mini section, but decided against it as this is not a common modification for Minis. Even though the kit is meant for Minis, it took a hell of a lot of work just to make it physically fit. I've not seen another Mini with this pedal box fitted, and there are very few running bias pedal box settups as it is. Was hoping to get advice from general race car enthusiasts who may be running a similar settup to me.

[Lotobear]

I'm not familiar with this set up at all, however i've had an issue with similar symptoms before on a MC with adjustable push rod where the brakes were locking on.

It turned out you need some 'lash' in the push rod to enable the pressure to be released back into the MC from the system when you release pressure on the pedal. I backed off the adjuster on the rod a few turns to achive a mm or two of slack and all was well.

[chris_rd92]

Just bumping this again as I'm still having problems. Things are slightly different now though, I've managed to fix the sticking front brakes, but the pedal is still terrible. I think I've narrowed it down to the rear brakes. Looking at the bias bar, it seems to there is much less resistance in the rear circuit then front, even with the bias adjusted all the way to the front, could this be a faulty master cylinder? Even though they're brand new?

Could I also request this be moved to a kit car based section or similar? It's just because this isn't a typical Mini problem, and this setup is much more common on kit cars and race cars in general.

Thanks for all the suggestions so far though, I appreciate it.

[guards red]

Can you not disconnect the rear lines and put a bolt in the rear brake master cylinder outlet to rule out the rear brakes, then same again for the front?