Allen Payne Posted June 22, 2002 Share Posted June 22, 2002 Can anyone help with a brake problem? The problem I get is excessive brake pedal travel after the car has been sat in between sessions during a track day. After a couple of laps the pedal is OK again and I don't have any problems on the road. The brakes are standard and I've bleed numerous times, using dot 5.1 fluid, but seem always get the problem once the car has been stood for a while with hot brakes. Anyone with any ideas / cures? Thanks Allen Link to comment Share on other sites More sharing options...
Jason Plato Posted June 22, 2002 Share Posted June 22, 2002 Are you parking the car with the handbrake on after the sessions ??? dave Link to comment Share on other sites More sharing options...
Allen Payne Posted June 22, 2002 Author Share Posted June 22, 2002 No - Whats a handbrake? Link to comment Share on other sites More sharing options...
felix.klauser Posted June 23, 2002 Share Posted June 23, 2002 If you're running standard rear brakes, try giving the handbrake lever a few good tugs but don't leave it engaged. This will ensure that the slack in the handbrake mechanism inside the caliper is removed. Otherwise your hydraulic brake actuation first has to overcome this slack, leading to long pedal travel. Edited by - felix.klauser on 23 Jun 2002 05:33:02 Link to comment Share on other sites More sharing options...
Peter Carmichael Posted June 23, 2002 Share Posted June 23, 2002 Sounds more like a problem with the heat in the rear brakes. Are you doing a warmdown lap. Peterid=teal> 253 BHP K-seriesteeth.gif, no gearboxbum.gifid=red> Link to comment Share on other sites More sharing options...
Allen Payne Posted June 24, 2002 Author Share Posted June 24, 2002 Peter - Yes I always try to drive a slow cool down lap, although I have noticed they do get hot. Why would hot brakes cause long brake pedal travel? Would different pads make a difference to this problem? Thanks Allen Link to comment Share on other sites More sharing options...
ashaughnessy Posted June 24, 2002 Share Posted June 24, 2002 Just to reassure you that this is a fairly common occurrence. What kind of car/brakes do you have? De-dion or live axle? It is common on my live axle car and I know other people have reported this behaviour in the past. Anthony Link to comment Share on other sites More sharing options...
Allen Payne Posted June 24, 2002 Author Share Posted June 24, 2002 It's a 1400SS DeDion with standard brakes. Regards Allen Link to comment Share on other sites More sharing options...
David Hopkins Posted June 24, 2002 Share Posted June 24, 2002 First point is that actuating the hbrake does nothing to take up any clearance between pad and disc other than on the application in question . The brake only adjusts on the service brake . What you maybe experiencing is 'coning' of the disc . Basically as the disc gets hot it distorts , it cools down it returns to its normal shape but the pads will have remained in the position they were when the disc was hot , hence the increased running clearance ( resulting in increased pedal travel) . First application once cold restores the intended clearance. You could check this with a feeler gauge cold v hot to see and also see where the biggest problem is fronts or rears. From your earlier comment I guess you dont have a hbrake ?? Without knowing more its difficult to determine why you are in the minority with this problem ( but maybe you aren't ). The only other thing is that maybe you have too much clearnce in the wheel bearings so when you subject the car to high cornering forces the disc is forcing the pads back , I dont think its this though as you would experience this dynamically . What is the exact brake spec of your car , calipers discs etc and the make . Link to comment Share on other sites More sharing options...
Peter Carmichael Posted June 25, 2002 Share Posted June 25, 2002 If the rear pads are getting thin, you may get a lot of heat soak back to the caliper. Many underestimate how hot the rears get, but they don't get any airflow. When you leave the car sitting, everything heat soaks. I don't know very much about the handbrake mechanisms, but if there is a malfunction could it cause excessive pad drag? A sticky piston might cause excessive pad drag which would cause overheating. "coning" diska sounds rather exotic and unlikely unless there is a problem elsewhere in the system. Peterid=teal> 253 BHP K-seriesteeth.gif, no gearboxbum.gifid=red> Link to comment Share on other sites More sharing options...
Allen Payne Posted June 25, 2002 Author Share Posted June 25, 2002 I do have a handbrake, I just never use it - the rear pads a new'ish standard ones. I'll check for play in the wheel bearings and try and check for the rear brakes pads dragging on the disks. Thanks Allen Link to comment Share on other sites More sharing options...
felix.klauser Posted June 30, 2002 Share Posted June 30, 2002 > First point is that actuating the hbrake does nothing to take up any > clearance between pad and disc other than on the application in question. > The brake only adjusts on the service brake . I'm not sure I agree with this statement. On a track day it's easily possible to get significant wear on the rear pads. If the handbrake isn't applied the pistons will retract to their pre-wear positions giving a long pedal. Period application of the handbrake will allow the handbrake mechanism in the caliper to "catch up" to the new pad thickness and remove the slack. Link to comment Share on other sites More sharing options...
David Hopkins Posted June 30, 2002 Share Posted June 30, 2002 The rear caliper adjuster only works on the service brake . The caliper only adjusts when the running clearance exceeds the design intent which is essentially the clearance between the threaded push rod and the nut in the piston . The caliper also has a load insensitive feature which means it will adjust only at lower line pressures basically to ensure you dont get overadjustment . Applying the handbrake will ofcourse reduce pedal travel but when you release it the clearance will return to clearance you had before the handbrake was applied . The handbrake is operated by a simple cam and strut design . I know all this is true because I work in the company that designed and developed them , originally for the Ford Scorpio in 1985 . Link to comment Share on other sites More sharing options...
felix.klauser Posted June 30, 2002 Share Posted June 30, 2002 Hmm, we have an apparent contradiction here. Your knowledge of the detailed innards of the caliper speaks for itself but I have several times solved a long pedal travel problem by applying the handbrake several times. Why would that then still make a difference? Link to comment Share on other sites More sharing options...
EFA Posted June 30, 2002 Share Posted June 30, 2002 David 1 Felix 0 Sorry Felix!tongue.gif PC is actually correct however, the discs dissipate heat into the pad and the material expands, causing the long pedal on first application. The fluid can also get clsoe to boiling point at this time which in itself givges a spongy pedal. These problems are particularly so if you use the std master cyl which has a huge amount of latency in its stroke, which emphasises the problem. Solution is not to brake to an absolute standstill when you park up between sessions, try to roll to a halt. Also do a cooling down lap with minimal braking to get the heat out of the discs. Fat Arn Visit the K2 RUM siteid=red> See the Lotus Seven Club 4 Counties Area Website hereid=green> Link to comment Share on other sites More sharing options...
David Hopkins Posted July 1, 2002 Share Posted July 1, 2002 I am not sure why it seems to solve the travel issue you spoke of but it maybe the following , - When you apply the handbrake you close down the running clearance between pad and disc . Since the caliper is single sided when you release the handbrake the pad nearest the piston will retract especially if it had sticky damping shims , the outer pad though will not , that is not until the disc 'knocks' it back , centralising the caliper body . This may not sound too good , the residual torque generated though is negligible . If you apply the brakes sensibly ( lower line pressures, on a warm down lap the self adjuster will take up any wear that has taken place . If you are using high line pressures on the track which i guess by definition you are then the adjuster won't work as it has the load insensitity feature built in , ie a clutch inside the piston . This is why when you change pads you have to apply a force to the piston when winding it back in to close the clutch . Otherwise the piston will just rotate and not travel back down the threaded adjuster pushrod inside the caliper . The other cause could be 'coning ' as I mentioned . This combined with the fact the adjuster doesnt operate at high line pressures could explain things . Next time try applying the brake to generate 0.1to 0.2g decel ie light 'check' braking as if you were scrubbing 10mph off in traffic . Let things cool down and then see what transpires . Link to comment Share on other sites More sharing options...
David Hopkins Posted July 1, 2002 Share Posted July 1, 2002 More thoughts , Why do you think its rears ? If you apply the handbrake statically and then apply the service ( foot ) brake you will notice the pedal travel is significantly reduced . You should isolate whether the problem you decribe is f , r or both by checking the clearance between pad and disc statically when cold . Do your track run let everything cool down and then repeat the measurement . In terms of pad swell due to heat , the rear material if using std stuff ( marked D8106/1 ) is specifically developed ( very low swell with temp rise ) for rear handbrake calipers . Imagine where your car would end up if you apply the brake hot , everything cools down, pad swell dissappears ...... along with your clamp force and the car . This material does suffer from high wear rates at high temps so this combined with no adjuster function due to hi pressures could explain things . Link to comment Share on other sites More sharing options...
David Hopkins Posted July 1, 2002 Share Posted July 1, 2002 More thoughts , Why do you think its rears ? If you apply the handbrake statically and then apply the service ( foot ) brake you will notice the pedal travel is significantly reduced . You should isolate whether the problem you decribe is f , r or both by checking the clearance between pad and disc statically when cold . Do your track run let everything cool down and then repeat the measurement . In terms of pad swell due to heat , the rear material if using std stuff ( marked D8106/1 ) is specifically developed ( very low swell with temp rise ) for rear handbrake calipers . Imagine where your car would end up if you apply the brake hot , everything cools down, pad swell dissappears ...... along with your clamp force and the car . This material does suffer from high wear rates at high temps so this combined with no adjuster function due to hi pressures could explain things . Link to comment Share on other sites More sharing options...
Allen Payne Posted July 1, 2002 Author Share Posted July 1, 2002 Thanks for the suggestions. I'm at Oulton Park on Friday and will pay more attention to the cooling down lap and also try some very light braking to take up any wear. Allen Link to comment Share on other sites More sharing options...
Andrew Dent Posted July 1, 2002 Share Posted July 1, 2002 Allen Cured my 'long brake pedal' with a new master cylinder. A bit expensive for teh large on, but a great improvement. At least its easy to fit. Link to comment Share on other sites More sharing options...
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