Aussie66Fastback

Hi all

last time i took my falcon to the track, I had a bit of a brake issue. Started out ok then the pedal got softer and closer to the floor as things warmed up. Overheating I thought but the car was still pulling up ok.

Last weekend i started it up and drove out of the shed. Normal brake pedal. Sat there for 20 mins or so, revving and idling to warm everything up. Went to put it back in the shed having driven nowhere and pedal is soft almost to the floor.

wtf? Its gotta be master cylinder right? (btw the mc is nearly brand new?!?!)

.boB

Not necessarily. You probably overheated and glazed your pads.

Remove the pads, and lightly resurface them with a little 150grit sandpaper.

Use a sanding disc in a drill motor and resurface the brake rotor.

Then bed the pads in as if they're new.

With that much heat, you should probably get some fresh fluid into the calipers. If you're going to the track again, use at least a DoT4 fluid. DO NOT use a DoT5 synthetic fluid.

andrewmp6

Bob pretty much hit the nail on the head.Get you a brake fluid tester its little paper strips you dip in it and they change color.Its to see if your fluid is ok or not.If your trying to race it any way you can get away with bigger brakes or would that mean you gotta change a lot more.I know aus has some tough laws on modding a car.

Aussie66Fastback

hey bob, what you said is what i thought i'd done, but this time it happened without driving the car anywhere.

I just started it up drove out it 10 yards out of the shed - good pedal

20 minutes later, warmed up engine but having not driven anywhere - soggy pedal.

So, i figure its got to be one of...
1) master cyl bypassing.
2) brake fluid bypassing in one of the calipers some how?!??! there's no leaks
3) somehow got air back into the system and I need to bleed again, but how would this occur?

.boB

Once the pads and rotors have been glazed, they're not going to work well until you fix it.

When fluid gets overheated, some of the dissolved gas will percolate out. It often does not get re-absorbed into the fluid. Gas is easily compressable. You need to get some fresh fluiid in there.

Address those two issues first before doing anything else.

There's no way for fluid to bypass the calipers, since there's nothing to bypass. The caliper is simply a chamber with an expandable wall.

It can "bypass" the MC plunger though. That happens when a seal fails. The only way to repair it to rebuild the MC. This is unlikely; but certainly possible.

Aussie66Fastback

ok, thanks bob.

I'll give that a try and let you know what happens.

appreciate your help

kalli

do you have power assist? if you do then unplug the vacuum hose gpoing into the canister of the power assist. push a welding wire to the bottom of it (without piercing the membrane). If you have brake fluid on the tip of the wire then you kow you're loosing pressure through the pushrod.
But since you have the problem with engine warm I guess you're boiling your brake lines.

it is not neccesary in front at engine but might well be at back of car with a hose/line going close to exhaust system.

brakefluid is hygroscopic so if you have pulled a bit of water that would lower the boiling point of the liquid giving you all sorts of hassle. Did you try exchanging all brake fluid? (NO DOT 5 as mentioned earlier!)

jlg2002

After 30+ years of doing brakes, I have yet to see the pedal go to the floor for glazed brake lings/pads for the conditions quoted here. You have either a bypassing MC or air in the lines. Re-bleed the system (go slow when you pump them so you don't aerate the fluid) and if the pedal continues to drop and after you've confirmed no apparent leak points, get another MC. (Don't forget to look at your rubber hoses as source of air ingression into the system). I've never seen a power booster fail and cause a low pedal, they always fail and cause a hard pedal in my experiences.