MartinM 0 Posted March 2, 2006 As topic title really. Under medium to heavy braking. Changed wishbone bushes last night (both sides) - no change Checked subframe mountings - absolutely solid Laser checked tracking - within tolerance Discs - ATE power discs - <2000 miles - look fine, zero runout Pads - Mintex 1144 - <2000 miles - look fine Put car on MOT brake test rollers - LH front brake much more efficient (nearly leapt leftwards off rollers!) - can't really tell if it's overefficient LH front or weak RH front Checked calipers and pistons both sides - slide easily, no resistance on pistons Bled brakes (admittedly, the RH one only) - no air trapped Suspects left.... Master cylinder? Likely? Any checks I can make before blindly changing it? Top mounts? Unlikely surely? Flexible hoses? New 15 months ago. Am at a total loss Oh, by the way, got a road rally in 50 hours time! Would like a fix before then....... Share this post Link to post Share on other sites
jonD6B 0 Posted March 2, 2006 What car is it? How are the brakes split? Any rattles or knocking? Share this post Link to post Share on other sites
MartinM 0 Posted March 2, 2006 1.6 Gti - all completely standard. 3 port master cylinder - two to front (one each side), one to rear (pressure reducing valve then to splitter to each rear drum) MOT brake tester shows it's nothing to do with rear brakes - back wheels weren't even going round when it pulled left..... No particular knocking noises.... Share this post Link to post Share on other sites
jonD6B 0 Posted March 2, 2006 (edited) I assume you have checked the N/S spring and damper? It sounds like an iffy caliper but if you're sure they're both good I'm struggling too. Low pads and discs can occasionally cause this affect but again with less than 2k these should be o.k. Edited March 2, 2006 by jonD6B Share this post Link to post Share on other sites
MartinM 0 Posted March 3, 2006 <<I assume you have checked the N/S spring and damper?>> Not specifically. What to check? I'm still working on the basis that the problem is with the RH brake - the MOT brake test rollers show quite a large imbalance between the LH (stronger) and RH braking efficiency: - I can't think of a failure mode that would give better braking on the LHS than normal (and if there is one, can I have it on the RH side as well please!) - I'm assuming there's no LH suspension problem that's causing such a massive mis-alignment on the LHS such that it looks like it's a brake efficiency problem. When it 'went left' on the rollers under braking, I didn't look to see there was any major toe out/wheel movement occurring. In hindisght, I should have looked I suppose. Next plan is to swap the pads L to R - can't really see it will cure it, but it has to be done. Can't think of a failure mode in the master cylinder that would give a differential hydraulic pressure in the LH and RH front brakes - and don't particularly want to change it 'just in case'. Any (easy) tests that I can do to measure any differential pressure? Not considering the rear brakes/pressure reducing valve at all - brake roller test showed the fault with the rear wheels stationary - or could it be *something* in the rear system? Any more for any more - 36 hours until the event Share this post Link to post Share on other sites
jonah 1 Posted March 3, 2006 It's definitely not the master cylinder. It's just not possible for it to give different pressure to front left and front right when they're both on the same hydraulic circuit. Only things I can think of are a problem with the friction surface of one of the pads (bit of oil contamination that you can't see?), or the caliper sliders... you said the pistons were free, are the sliders also free? The sliders on the 1.6 calipers can also wear and cause the pads to run at an angle to the disc, check the pads for uneven wear (tapering from one end to the other), this could possibly cause what you're seeing in a severe case. New pads should solve it for a while but they'll wear the same way again eventually. btw is it the Gremlin you're doing this weekend? Share this post Link to post Share on other sites
MartinM 0 Posted March 3, 2006 ...you said the pistons were free, are the sliders also free? btw is it the Gremlin you're doing this weekend? Sliders checked and free Plan tonight is: - examine and swap pads L to R. Road test. - swap front wheels L to R. Road test (unidirectional tyres so might be interesting) - jack up LH front wheel, chock all other wheels, start engine, engage gear, clutch out, brake lightly (standing clear!), see if wheel moves - is this a stupid test, safety-wise, and would it show anything anyway????? - get used to dialling in some 'right hand down' when braking - anything else to test/look at? Rally of the Moors, Exeter Motor Club - first round of the ASWMC (novice) Road Rally Championship - was 2nd o/a in last year's championship, want to win this year! Share this post Link to post Share on other sites
jonah 1 Posted March 3, 2006 - jack up LH front wheel, chock all other wheels, start engine, engage gear, clutch out, brake lightly (standing clear!), see if wheel moves - is this a stupid test, safety-wise, and would it show anything anyway????? Yes, and No. The diff will put equal torque to both wheels, and if the LH brake really is stronger than the RH, then the braking torque will cancel out the engine torque more on the LH wheel than the RH wheel. So the RH wheel will receive a net forwards torque that will try to pull the car off the jack. If you're trying to find suspension movement by doing this, then you probably won't see it anyway, because there won't be any longitudinal force on the front wheels like there is under real braking. get used to dialling in some 'right hand down' when braking Are you actually having to steer to the right to keep the car in a straight line? Or just hold the wheel still while it tries to tug to the left? If it's the first one then it's probably a suspension problem... if it's the second then it's probably the brakes. Other thing to check on suspension is bottom balljoint clamps... the bottom of the hub carrier can go bellmouthed if the clamp bolt works loose, allowing the balljoint pin to move around. Good luck tracking it down... let us know what it was! Share this post Link to post Share on other sites
RichF 1 2 Cars Posted March 4, 2006 Hub carrier to wishbone joint - as mentioned above the hub carrier 'socket' can be so worn that it doesn't hold the ball joint properly no matter how much you tighten the clamp. Is is me or have we had a spate of topics with the same symptoms recently ? This is not the first time that I've given this answer, also not the first time I've agreed with Jonah about exactly the same point! Share this post Link to post Share on other sites
MartinM 0 Posted March 4, 2006 Well, the mystery deepens.... Swapped the brake pads and the wheels over (fronts now rotating backwards - Bridgestone RE720's) thinking I really was wasting my time and guess what? - LH pads were 12.5mm, RH were 14mm - It now pulls to the right - Had it back on the MOT brake test rollers - RH brake now more efficient, but not as much as the left hand one was - Pads are making quite a lot of noise at the moment (only done 5 or so miles since the swap) Shouldn't have changed two things at once, of course, but time is running out and I'm going with it as it is! I'll do some more tests, depending on how we get on tonight. Thanks, for now, to all PS definitely not hub carrier to wishbone joint.... Share this post Link to post Share on other sites
C_W 3 1 Cars Posted March 7, 2006 Are the pads glazed on one side? Try sanding them down and refit them. Share this post Link to post Share on other sites
MartinM 0 Posted March 9, 2006 Ok....did the event - 10th o/a, 1st in class, which was nice - and despite driving through a shooting alley (see front page of this week's Motor Sport News and a picture of us on page 3 ), was unremarkable in terms of braking. Pulled up in a straight line, no premature locking etc etc Now, 300 miles after swapping the pads and wheels, it starting to pull left again. What's going on PS need a new front valence (see MSN picture) and a windscreen - anyone got either/both? I note MSN used the word 'allegedly' a lot - I can assure you we were most definitely shot at with live ammunition Share this post Link to post Share on other sites
