Originally Posted by
daveyboydave
Thanks for the thoughts Dan.
The setup is R33 GTR fronts and standard rear calipers, with the BM57. This is what came on the car when I bought it about 8 years ago (I didn't even realise the BMC was non-standard until I refurbed it last month).
I can see how this may not be absolutely optimal, and one of the reasons I took the opportunity to fit the proportioning valve is to be able to tweak the balance to refine it if need be. (It might be that at some point I shim the internal BMC proportioning valve to allow the external valve to be able to have more control and set a higher pressure to the rears to compensate, but that's future Dave's problem, I just want to get it working as it was for now!)
That said, it's been absolutely fine all this time, on multiple track days. I've taken it to the same MOTer every time, and he knows what he's talking about (FCM Motorsport), and this is the only time it's had a problem with the rear being under-braked on the rollers. Which is why I'm so certain it's something I've done or fitted during the ABS delete and general refurbishment, either through defective parts or idiocy on my part!
With regards the splitting of lines, with the ABS BMC there is only a single outlet for front as well as the single for rear, so both pairs are fed via a T-piece and therefore I don't think it's an issue. The amount of fluid flow (noting pressure is the key point not mass flow rate, but whatever) is the same as before I've done the recent work, I've just removed the ABS pump and about 50% of the line length from all the plumbing in between.
Last night i refitted the BMC and re-bled the system. Took it for a test drive and the pedal feel was ok, could be firmer. Braking was reasonable but I could easily lock up the front left wheel and no other. Feeling the disc temperatures afterwards, both fronts felt reasonably equally hot and the rears had some temperature, so all 4 brakes are doing something at least....
I'm starting to think the issue might just be poor bleeding. Until last night, but since the ABS delete, I was getting some creaking from the front left caliper, which I put down to maybe a sticky piston. After last night's re-bleed the creaking was gone and it was the corner that locked up. Makes me think there was some air still in one side of the caliper, and the asymetric clamping force was causing the creak.
Anyway, chatting with Jon at FCM, he says when flushing and filling a new system he might put 2 litres of brake fluid through. I've probably used about a litre so it might just be there's still air trapped at certain points causing certain corners to be weak. (I'm also dealing with the age-old problem of a wife who gets bored pushing the pedal for the 50th time.....)
I've just ordered some Goodridge speed bleeders and some more brake fluid, so I can bleed single-handedly to my heart's content without incurring my wife's wrath. Plan is to fit the those and absolutely bleed the shit out of everything, like 10 times, until I can completely rule out air as a root cause. Perhaps not a surpise that with an entirely new braking system, it's going to take some perseverance to get every last bit of air out. The BMC is almost certainly good, the calipers are good, the lines are just lines, and the Tilton proportioning valve should only ever fail open, so air feels like the most likely remaining candidate.