Weird brake problem!

#1

After my recent rebuild project I’m having a weird problem with the front brakes. I had the system completely dry of fluid for some time as I installed a new master, rebuilt all 4 calipers, changed the brake lines and installed a new clutch master and slave. When I got it all back together the clutch bled out just fine, as did the rear calipers. The front was a little weird though, while bleeding the front brakes when I open the bleeder the pedal doesn’t go all the way to the floor it only goes about half way?? I had the car out at CMP this past weekend and the brakes felt okay, but then I drove my Dad’s car and his brakes were much better than mine. I could push his car farther into the corners and get on the brakes hard and not engage abs, while my car I had to brake sooner and I was engaging abs every corner.

I’m thinking maybe it is my abs pump, but in my experience they usually just fail and don’t let any fluid through. I’m getting fluid to the front calipers it just doesn’t bleed normally, pedal only sink about half way. Pump brakes hold, then open bleeder and pedal sinks and fluid is pushed out the bleeder then it just stops about half way??

Any ideas???

#2

Kind of a reach, but I had what I recall as similar symptoms a few years ago, and swapping the MC resolved it. Was the MC you installed actually new? Any chance you have another lying about, even if you only borrow long enough to see if the bleeding anomaly is resolved?

It doesn’t seem like it would be the ABS pump unless a valve is stuck partially closed, since it is happening with both front brakes. That would be 2 valves, wouldn’t it?

#3

Since it’s both front brakes, I’m going with Chuck. The ABS pump has separate channels for each front wheel so it’s less likely to produce the same symptom in both front wheels. The only thing shared by the front wheels is the MC.

That’s a plausible cause for the bleeding issue but the ABS being triggered under heavy braking is harder. If the car is braking straight then both fronts are getting equal braking force. If ABS is triggering it’s because a wheel is stopping. If you can stop the wheels then the brake calipers are getting as much pressure as they should. Hmmm.

What if, in addition to the mystery front bleeding problem…the rear brakes aren’t getting much pressure due to the same MC problem? That would mean longer stopping distances and a tendency for the front wheels to lock up. It’s hard to understand how a bad MC would cause that mix of symptoms, but the symptoms do all fit.

BTW, back when I was playing musical boosters a couple months ago I came up with a way to swap MC’s w/o having to bleed the whole system. The trick is to move the air bubbles towards the mc, not towards the calipers.

  1. Make sure all brake pads are really thin. The thinner the better.

  2. Pull off old MC w/o letting brake lines drip much.

  3. Put on new MC, reservoir and fill 1/4 to 1/2 full.

  4. Have assistant go to each corner and compress slave cylinders into calipers. Once slave is compressed, be sure it remains compressed. When the asst gets to the 2nd rear wheel you should see fluid w/o bubbles come into the reservoir. Same for 2nd front wheel. Since the rear cylinders are smaller, that’s the harder of the 2 pipes.

#4

The master cylinder I used was off my street car, It had been replaced about a year ago. I bought a E31 master to save some money and installed that one on the street car. The master I used was in known good working condition. It did sit on the work bench wrapped in a bag for 3 or 4 weeks before I got around to installing it.

The car was braking straight. I thought more about it this morning, and I think the main difference between the two cars was tires. I was running a really crappy set of RA1s and my Dad had on a set of almost new Nitto nt01s in 225. That explains the abs coming on but not the bleeding issue.

I guess I’ll try to find a master to test if the bleeding issue is resolved.

#5

Tried to bleed the system again today and nothing really changed even after running the car hard at CMP. New brake master is ordered, and hopefully that will solve the problem. I’ll update once I install the new master.

#6

New Master cylinder arrived today and I got it installed tonight with no change. The front brakes still don’t bleed properly. The pedal still only sinks about an inch. I get fluid through the bleeder, but it won’t sink all the way to the floor like it should.

List of parts I recently replaced/overhauled on the braking system.

4 rebuilt by me calipers
all 6 new ss brake lines…I switched one of the old brakes lines onto the front tonight with no change…so doubtful this is the problem
new bleeders on all 4 calipers.
new master cylinder twice…
new pads/rotors but that shouldn’t make a difference
new brass guides

While it was all apart I also replaced the clutch master, the slave, and the clutch line.

Help with ideas…I’m stumped

I took the car to CMP two weeks ago with this issue and the brakes felt okay. I’m scheduled to race at RA this weekend and I’m really not happy with having a looming brake issue that I have no idea of the cause. If I can’t get this fixed before hand I’m inclined to get my annual done then put the car back on the trailer.

#7

The ABS pump is 2 “in” and 3 “out”, correct? That is to say the LF and RF are a single pipe going into the ABS pump, but two lines coming out. If that’s the case, the problem could be some bizzare failure inside the ABS pump “before” it separates into 2 front channels. I’d be happy to bring a spare pump to RA.

To make that swap easier, you might want to torch your brake lines fittings at the pump now, before you load up. Loosen them and then retighten them. That way they’ll be easy to remove at the track. Otherwise, bring a torch to the track.

#8

[quote=“Ranger” post=72765]The ABS pump is 2 “in” and 3 “out”, correct? That is to say the LF and RF are a single pipe going into the ABS pump, but two lines coming out. If that’s the case, the problem could be some bizzare failure inside the ABS pump “before” it separates into 2 front channels. I’d be happy to bring a spare pump to RA.

To make that swap easier, you might want to torch your brake lines fittings at the pump now, before you load up. Loosen them and then retighten them. That way they’ll be easy to remove at the track. Otherwise, bring a torch to the track.[/quote]

I’ve replaced the ABS pump before so the lines shouldn’t be a problem. I’ve got a spare pump too that I could swap in tonight.

I talked to Bob Shields about it this morning and he thinks it is normal and the pedal in the front shouldn’t drop as far as the back once the back is properly bled. I’m not sure because Pops is always in the car when we bleed brakes and I’m always working the bleeder. It is possible My Dad is losing his mind and this is a normal issue. He is getting old!!!