Bimota Guy

For 2 race weekends and a track day (5 days on track) I have been trying to solve a low brake pedal problem. I start out with a high pedal and after a couple to a few laps it gets low. It typically happens from one corner to the next, but sometimes it takes a few corners to develop. Still good bite, although if you get a bit of fade it can come close to hitting the floor and you lose bite. You can pump it back up. I would pump lightly a few times with my left foot entering a slow corner after a long straight.

After each on-track session (now about 15!) I get air when bleeding, mostly from the rears, and mostly the right rear. And normally it takes several cycles (manual bleed with someone pressing the pedal) before you see any significant air come out. Not just bubbles, but gaps in the fluid can be seen in the bleeder bottle hose, the largest about 1/2-3/4 inch.

It started after I swapped out all 4 calipers on the car. I installed new C6 calipers up front and C5 calipers in the rear and flushed the fluid. I used a power bleeder (w/o fluid in it) to flush the system. I may have gotten air in the system as I did see the master get quite low at one point while working on the rears. But what you see below the cap is the well for the fronts, which had a bit of fluid visible, but I am not sure what the well for the rears looked like (i.e., I do not know if it went dry and was sucking air.)

This does not appear to be a pad knock back problem due to air being found each time. I have gone through rotors and pads, so I am taking them out of the equation since this happens on new and old rotors, as well as new and old pads.

After the two race weekends of chasing this problem I had the system flushed by a dealer using the Tech 2 so the ABS system got flushed as well. They found no air (I had bled them manually after the last track session and got some air from the rears.) This was before yesterday's track day where the problem occurred again. At the track day we replaced the rear brake (flexible) lines and the rear calipers. The pedal went low again.

Any experience with such a problem? TIA...

sleeper02Z06

Before you do anything else, jack the car up, look above the diff. You can see the fittings where the hard lines in the tunnel connect to the rear brake lines that run down around the diff. I have seen those leak/loosen up before. Rare, but just ruling out a small variable before I tell you what i really think it is......

bad master cylinder assembly. Although I have not experienced this in any of my GM vehicles, my wife use to daily commute in a high milage Acura Integra. It exhibited the exact same problems that you are describing, except it was doing that in daily traffic. At stoplights I could slowly press the pedal to the floor. Replaced the master assembly forward of the booster and the issue disappeared. Worth a shot.

Joe

trackboss

One thing to make sure is that the air you see is not coming from the bleeder screw or the hose where it seals around the nipple. It is not uncommon to get air bubbles in the from that area and then be led to believe the air was coming from the actual brake lines forward of the caliper.

mousecatcher

Based on the circumstances, my thought is that you are introducing air into the system while the car is running. It must be a big leak if it only takes a couple of laps. Assuming all the fittings you replaced are fine, something like the above is where I'd look next.

Perhaps you can reproduce it when the car is parked, just by simulating a few laps of normal braking.

kmagvette

If I read your description correctly, you purged the system of all brake fluid with a dry power bleeder. If so, you pressurized your system with air. You may have introduced air into the ABS system. During a track session you may be engaging ABS and injecting air into the system that way.

Perhaps a TECH2 bleed is in order.

MySR71

As was suggested before, you might want to check your master cylinder. I was taking to someone a year or so ago and they said that when their master cylinder started to leak, they saw air when bleeding their rear brakes. He said that he could bleed the brakes, then leave the car for a few days then bleed the brakes again and find air in the rears again.

Since you have to bleed so much fluid before finding air, that makes me also suspect that it is the master cylinder as you have to get all the good fluid between the leak at the master cylinder and the caliper out before seeing the air.

I'm not sure how to test the master cylinder for leaks to get confirmation that it is the problem before replacing it.

Food for thought.....

Bimota Guy

Tech 2 bleed was done after a regular bleed once again got air and pedal was back to normal height...no air.

I forgot to mention several of the things suggested...

• I did check the fittings above the diff...dry. Are there other fittings inside the torque tube tunnel?
• I looked at all fittings I could see (e.g., master and ABS unit)...dry.
• Once bled, I can press hard on the pedal for over a minute and it does not fade. This with car running or not. Suggests master not leaking.
• I pulled the master off the booster and it is not leaking at that end.

0Randy@DRM

[QUOTE=Bimota Guy;1570038763]Are there other fittings inside the torque tube tunnel?[/BQUOTE]


NO

Randy

froggy47

Can you put the flourescent dye in brake fluid?

mousecatcher

no, i don't think so. i've looked for dye that is compatible with brake systems for a long time and have never found anything.

0Robert@SPEEDSOUTH

Do you have speed bleed valves? I've seen them leak, they were machined wrong.

Krispy

I had the same symptoms with a 87 Honda CRX, even everything on the master cylinder checked out fine like you've said in your case. While I didn't think the MC was the problem I changed it along with the brake booster anyway, good thing too because that was the problem. MC's are mysterious sometimes....

Bimota Guy

No. Had them on the old calipers, though.

jazzmk1

Just a shot in the dark, could it be the fluid is boiling in the lines?

mousecatcher

maybe one of the rear calipers has a bad seal

Bimota Guy

Replaced them both this past weekend at the track. Did them one at a time. Same problem in each session after each swap.

Before that I took them apart (the new ones) and saw no evidence of any leaking. The seals felt smooth and the pistons were tight when sliding them in.

Bimota Guy

Possible. But why all of a sudden?

Sidney004

You indicated that you had changed calipers(were the C6 calipers brand new or previously raced?) and it occurred, if they were used and raced, you may have mechanically spread(thermally damaged) calipers. Corrosion on the bleeder seat has caused problems for me in the past(but again used calipers and seepage around bleeder.)

jazzmk1

Has fluid level in the master cylinder changed when this happened? Are you using a high temp fluid, are the calipers "floating" right or are they just working from one side maybe the caliper is not clamping evenly? I'm not a mech. but this seems as if it is mechanical, too much heat, or your master cylinder is bad. I don't know if that helps, troubleshoot your system. Mark

Bimota Guy

All 4 were new.