Mechanic's Corner - brake problems

#1

I am having anti lock brake problems:

After about running 10 minutes or as soon as my brakes heat up the anti lock brake stops working. In fact the brakes will lock up under medium braking not only threshold braking. So braking in high speed turns is crazy, not to mention all the flat spots on the tires.

Here is the history:

Mid Ohio - changed rear brake pads ht10, changed front pad to Performance Friction. problem first appeared.

Changed all 4 calipers at mid Ohio and put HT10’s on front. But clutch blew up first race on Friday so brake never got hot.

Went to Barber - with ht10 all the way around - went out about 5 times on saturday including race, no problem all worked well.

For some dumb reason I decided to see if the problem was the Performance Friction pads ( early in the summer I had used a set of PF pads and really liked them). So I put the same PF pads on I tried at nationals…bingo problem re appears another two tires flat spotted.

So now I have to get ready for Sunday’s enduro. I go back to the HT10 in the fronts, same I used on Saturday. After 10 minutes the brakes lock up under medium braking. Needless to say I ran for about 25 minutes locking up the brakes everywhere even being gentle on them. Once the tires got to square I called it quits and came in.

what is going on?

Carlton

#2

You sure it’s only after they get hot? I saw them lock up at least once on a formation lap…I think that would’ve been Sunday’s race.

#3

Mike, perhaps they did once, but when we raced the first few laps together I did not have the problem, than after about 4 laps they locked up all over the place.

Carlton

#4

Could it be that the caliper pistons are sticking (or at least not retracting well) when they get hot?

#5

Carlton,
Does your ABS light come on? SOunds like the ABS pump could be on it’s way out.
-Vic

#6

You can get a fair amount of lockup without flatspotting RA1’s you must be getting into them pretty good.

I’d look into swapping out the relays first to see if it makes a difference.

#7

I replaced all brake calipers and it did not fix the problem. I am not really getting into the brakes hard when they lock up, medium pressure and even light braking causes them to lock when they get hot.

#8

Have you tried disabling the ABS system?

#9

Sounds to me like it could be the master cylinder.

#10

Carlton - I think Chuck is right. You have replaced so much of the system already, why not go for it. I would also look into the brake booster. It pulls vacuum from the intake manifold. Make sure these lines are in working order as well. I had a Triumph that had a vaccum leak to the booster and it would lock up without notice.
Best of luck,
Ed