Binding Brakes; Trailer home instead of Track Day!
Discussion
My Peugeot 205 with recently installed 307 discs, calipers & 405 mastercylinder (up from 20.6mm to 22mm)had to be trailered home instead of going to Elvington this morning.
Commuting trips for the last 4 days have been fine, although the discs were warmer than I expected, so put it down to bedding in, (I could push it OK, but was certainly tighter than before) especially as it seemed to get better during the week. Today, on route from Nottingham to Elvington in York, again OK for 30 miles on the A614 & A1, but after 35 & braking for a roundabout, the brakes felt grabby. Accelerating out of the roundabout it felt hesitant & as I dropped off the throttle it was obvious they were binding. Not on fire, but very hot & smoke from both front calipers. The fluid in the mastercylinder was still cold.
Question is, why after 35 miles? I presume that the slight binding magnifies over distance & with increased speed, that makes everything get hotter which in turn expands the fluid & pushes out the caliper piston causing more friction?
Someone mentioned adjusting the rod that goes into the mastercylinder from inside the servo. If so, how do I adjust it & which way? Could be a lengthy process re-bleeding every time & then having to drive 40 miles at fairly quick speeds just to check! I'm convinced the problem is at the mc/ servo end of things.
Any bright ideas would be very gratefully received
Mark.
Commuting trips for the last 4 days have been fine, although the discs were warmer than I expected, so put it down to bedding in, (I could push it OK, but was certainly tighter than before) especially as it seemed to get better during the week. Today, on route from Nottingham to Elvington in York, again OK for 30 miles on the A614 & A1, but after 35 & braking for a roundabout, the brakes felt grabby. Accelerating out of the roundabout it felt hesitant & as I dropped off the throttle it was obvious they were binding. Not on fire, but very hot & smoke from both front calipers. The fluid in the mastercylinder was still cold.
Question is, why after 35 miles? I presume that the slight binding magnifies over distance & with increased speed, that makes everything get hotter which in turn expands the fluid & pushes out the caliper piston causing more friction?
Someone mentioned adjusting the rod that goes into the mastercylinder from inside the servo. If so, how do I adjust it & which way? Could be a lengthy process re-bleeding every time & then having to drive 40 miles at fairly quick speeds just to check! I'm convinced the problem is at the mc/ servo end of things.
Any bright ideas would be very gratefully received
Mark.
I've seen this problem a couple of times.
The problem occurs because the hydraulic circuit is normally open to the reservoir, but becomes a sealed system after you move the master cylinder the first fraction of an inch. When you take your foot off the brakes it remains sealed until the master cylinder returns that final fraction of an inch to open the port to the reservoir again.
There are various faults that can prevent the master cylinder from returning that last critical fraction of an inch, but the most likely one is that the pushrod to the pedal has been adjusted to take the slack out of the system.
If the system remains sealed then it will be fine initially, but as soon as the brake fluid starts to warm up and expand the brakes will start dragging, generating more heat and applying themselves even more firmly until eventually the whole thing locks up.
This is a problem that can occur spontaneously if the adjustment was marginal to start with and then the cylinder has worn or got gunked up. In these cases you may find that by stamping really hard on the pedal you can get it to 'rebound' far enough to open the port and release the pressure.
>> Edited by GreenV8S on Saturday 18th March 13:21
The problem occurs because the hydraulic circuit is normally open to the reservoir, but becomes a sealed system after you move the master cylinder the first fraction of an inch. When you take your foot off the brakes it remains sealed until the master cylinder returns that final fraction of an inch to open the port to the reservoir again.
There are various faults that can prevent the master cylinder from returning that last critical fraction of an inch, but the most likely one is that the pushrod to the pedal has been adjusted to take the slack out of the system.
If the system remains sealed then it will be fine initially, but as soon as the brake fluid starts to warm up and expand the brakes will start dragging, generating more heat and applying themselves even more firmly until eventually the whole thing locks up.
This is a problem that can occur spontaneously if the adjustment was marginal to start with and then the cylinder has worn or got gunked up. In these cases you may find that by stamping really hard on the pedal you can get it to 'rebound' far enough to open the port and release the pressure.
>> Edited by GreenV8S on Saturday 18th March 13:21
I agree with Peter on this one.
If you have changed the master cylinder and in particular to a different size then the rod adjustment will be the issue. Your Haynes manual may give a clearance figure but if not then you just need to ensure there is a small amount of endfloat in the rod.
I don't know the layout of the pipes into the master cylinder but you may be able to bring the master cylinder forwards from the servo without disturbing the brake pipes and requiring a bleed.
Have you also replaced the servo?
If you have then the problem may be the rod between the servo and the pedal. The same situation can occur in that the pedal comes back to a stop and the rod prevents the servo returning to its stop, which in turn prevents the master cylinder closing.
Steve
If you have changed the master cylinder and in particular to a different size then the rod adjustment will be the issue. Your Haynes manual may give a clearance figure but if not then you just need to ensure there is a small amount of endfloat in the rod.
I don't know the layout of the pipes into the master cylinder but you may be able to bring the master cylinder forwards from the servo without disturbing the brake pipes and requiring a bleed.
Have you also replaced the servo?
If you have then the problem may be the rod between the servo and the pedal. The same situation can occur in that the pedal comes back to a stop and the rod prevents the servo returning to its stop, which in turn prevents the master cylinder closing.
Steve
Thanks for the replies Chaps.
Sorry for the dumb question, but by "end float" I guess you mean the distances bewteen the end of the rod and the 'piston' inside the mastercylinder. I'll have to strip it all off & measure up. I guess it could do with a bleed at the front anyway as it got pretty hot.
Sorry for the dumb question, but by "end float" I guess you mean the distances bewteen the end of the rod and the 'piston' inside the mastercylinder. I'll have to strip it all off & measure up. I guess it could do with a bleed at the front anyway as it got pretty hot.
Cheers Steve.
The problem is exactly as you & Peter have described. The internal measurement of the new mastercylinder is 45.0mm, whereas the old one was 47.8mm. The 'missing' 2.8mm end float is clearly causing the brakes to come on when it all warms up.
The only problem now is that I can't undo the serrated locknut on the rod inside the servo & access is almost impossible. I may have to go back to the old mc for now as I don't want to remove the pedal box & servo now.
Thanks for your help.
The problem is exactly as you & Peter have described. The internal measurement of the new mastercylinder is 45.0mm, whereas the old one was 47.8mm. The 'missing' 2.8mm end float is clearly causing the brakes to come on when it all warms up.
The only problem now is that I can't undo the serrated locknut on the rod inside the servo & access is almost impossible. I may have to go back to the old mc for now as I don't want to remove the pedal box & servo now.
Thanks for your help.
Good thoughts. Studs probably could cope with a 2.8mm spacer, I'm not convinced I could make one though!
I couldn't get an angle grinder in, but a cutting disc on a dremmel might have worked if I'd thought of it!
The servo is a pig to get off; one of the four bolts is pretty much covered by the steering column, so I've put the old mc back on. Definitely no binding now, although I've not done 35 miles, but the pedal travel is longer than I would like. It'll have to do for now, but I'll have a go at it another time.
Thanks again for your suggetions.
Mark.
>> Edited by raceface on Sunday 19th March 22:28
I couldn't get an angle grinder in, but a cutting disc on a dremmel might have worked if I'd thought of it!
The servo is a pig to get off; one of the four bolts is pretty much covered by the steering column, so I've put the old mc back on. Definitely no binding now, although I've not done 35 miles, but the pedal travel is longer than I would like. It'll have to do for now, but I'll have a go at it another time.
Thanks again for your suggetions.
Mark.
>> Edited by raceface on Sunday 19th March 22:28
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