Moderator: martauto
So the rear bleeds fine? Ok, that pretty much narrows it down to rear pads, rear caliper pistons, or as you say a dodgey rear bias valve. Has the E30 got a load-sensing rear valve thing (it was in the air after all)?bumhole wrote:Well that's one thing I've learnt then! I didn't know the mc was arranged like that. I went for a quick drive last night and the brakes seem pretty good, obviously I'm only being gentle because of running new front pads in. So I assume the fronts work fine. Had a check and the lines aren't kinked, can't see any leaks ( line runs through inside of car) the only thing I can think is maybe glazed discs and pads or maybe that little restrictor to the back that I kept in. Something either wrong with it or blocked. Or I suppose the master cylinder could be faulty. Can't see how the restrictor could be blocked if I can bleed them fine.
Sweet!bumhole wrote:WMS kit looks ace though !!
Er....GeoffBob wrote:Bumhole, this is not the first time I have seen this question asked, and probably not the last.
I believe that the reason your rear brakes have become ineffective is as a result of fitting the larger front calipers (sorry Keri, no offence intended). What you need to understand is that the amount of force required for the pads to bite into a disc is a function of the caliper piston surface area. The larger the piston area, the less force (as a result of the fuid pressure) is required to brake the rotating wheel. It's the same idea as holding a heavy object firmly in your hand compared to between a finger and thumb. Just a finger and thumb will require a lot more force to hold the object without dropping it. The common or standard way of stating this is to say that a larger caliper provides "increased mechanical advantage".
Now, given that you have increased the size of your front calipers (larger area pistons), it now takes less force on the pedal to make your front brakes bite into the front discs, and hence slow the car down. Problem is, your tandem master cylinder divides the force you apply to the brake pedal between the front and rear fluid circuits and was designed by the OEM to have the front and rear brakes engage more or less simultaneously (slight front advantage actually). What is happening now is that your front brakes are engaging well ahead of your rears, with your rears lagging far behind.
At the extreme of this problem (at such point that the fronts are effectively locked up) you may find that you can't even get the rears to take at all because the fronts prevent you depressing the pedal any further.
Incidentally, this is the reason why many track and race cars employ dual M/C setups, where each M/C is sized individually according to the size caliper it is required to pump fluid to.
The only solutions that come to my mind at this time are that you either need to increase the area of your rear calipers by the same ratio that you increased the fronts, or else look for a tandem M/C from another car that runs a similar disc/caliper arrangement to that which you now have, which will bring your rear brakes "back on line", so to speak.
). Sometimes I type faster than I think (and I don't type very fast 
).
No problemo!GeoffBob wrote:Fair enough Keri. Had already realised and deleted the post before you replied, but you were obviously already in the process of reply.
With 4x1.25" pistons I would not expect to see what the OP has described (not enough of a difference from the original caliper) - so please ignore my post (that never was
Bumhole, there is a very nice explanation of how a tandem M/C works here
You are of course completely right that "average" 4-pots will have more area than the WMS ones, and mess with the E30 bias as a result - I'll leave the quote there if it's ok as it's a handy reference for anyone with missmatched 4-pots who might be having this kind of problem?With pleasure. Wouldn't be the first time I have seen someone with incorrectly sized calipers (and resultant bias problems), but I was clearly mistaken in thinking this was the case with the product you offer.keri-WMS wrote:No problemo!
That was where I first saw one, when I was looking at Gareth's old 205 1.9Gti.bumhole wrote:I don't think there is a load valve, only seen one of them on a peugot of a friends. It was a clunky looking spring device. I freed it off for him with a hammer and WD.
I'll have to defer to the OEM part gurus as regards the actual valve....but I wouldn't modifiy it, just service it etc.bumhole wrote:I'll reluctantly take the bias valve off, it's going to be a right game as the flexi hose joints don't swivel that I used, and I'll have to replace the olives. What do you reckon, throw it away? Drill a bigger hole in it?
Thanks for the help
Made me laugh when I first got a PM off you I must admit!bumhole wrote:I really wish I hadn't done my name as bumhole now!
Take the valve apart again and re-assemble with the internals omitted such the fluid can pass straight through. The fact that the brakes are getting hot tells you that the caliper is biting down (and thus doing work which = heat) , but not biting down hard enough due to low fluid pressure at the calipers.bumhole wrote:They will now in 2nd gear and they do seem a bit more powerful.
I drove the wheels for about 20-30 seconds with the brakes on and the discs got so hot i couldnt touch them. must be knackered discs and pads????
Interestingly, it's not so much a case of how much fluid you have to displace, but the force that is required to compress (to the required pressure) the fluid filling the space between (and in) the M/C and the caliper. Keep in mind that hydraulic fluid (unlike water BTW) is compressible. The travel of the pedal that you feel as you push your foot down isn't fluid being pumped into the caliper, its the fluid that is already there being compressed.e301988325i wrote:Geoff, I was concerned when I fitted my wilwoods up front that the rears would engage before the fronts because of the extra fluid that has to be displaced in the front calipers. . . Incidentally I upgraded the rear calipers shortly after to E30 M3 spec and I feel the balance has been kept.
) Took apart the bias valve like Geoffbob said and put it back together minus internal bits. Made a smidge of difference, it seemed slightly harder for the engine to keep the wheels turning. Today I bought some new discs and already had some new ferrodo pads. I put them on next and they seem to have made it again, slightly harder for the engine to keep the wheels turning but still they wont stall it. The handbrake will though! bonus! Possibility that new discs and pads need bedding in before they will bite.OK, the fact that you still aren't getting enough fluid pressure to the rear callipers (with the valve internals omitted) tells us that your valve is not to blame for any of this. Likewise, the fact that you can stall the engine by pulling up the handbrake tells me that the problem is not your pads or discs. Yes, I realise that the shoes are separate from the pads, but if a new pair of shoes can brake the wheel, then a new pair of pads most certainly will too (regardless of whether they are bedded in or not).bumhole wrote:Tonight (just finished
Any other ideas very much appreciated. What if the spacer I made for the master cylinder (with clio servo) was too short? What are the chances of the master cylinder being shafted?
) that the correct length for a spacer was 8-10mm,despite my measurements matching the op's at around the 25mm mark.
)and I found air trapped in the ABS pump. Yes, I think this would be informative. Keep in mind, however, that your front brake circuit would now be supplying one front and two rear callipers, so this is for test purposes only. If you drain the rear fluid circuit of your tandem M/C (in order to conduct this test) then the pedal will drop rapidly when you depress it until the secondary piston inside the M/C halts on its stops, at which time you will be able to pressurise the front brake circuit (now with rear callipers connected) as the pedal pumps the fluid in the chamber between the primary and secondary pistons. This assumes that the pedal doesn't bottom out on the bulkhead first (see that animation I keep banging on about here )bumhole wrote:I was thinking that if I pulled one of the front brake lines off the m/c and plug the rear in it's place.??
Might that work as a diagnosis?
Oh bugger!Sorry Dan...DanThe wrote:More like 20 - 22mm long Malcolm! I wouldnt have said 8 or 10
Sounds to me like the spacer is wrong
it's me age... 
daimlerman wrote:Oh bugger!Sorry Dan...DanThe wrote:More like 20 - 22mm long Malcolm! I wouldnt have said 8 or 10
Sounds to me like the spacer is wrongit's me age...
That must be why my pedal is still a little 'long'....
