Brakes not binding hard enough

#1

I had some really strange brake problems at Sebring last weekend. Once the brakes got hot, say lap 3, I had to press on the pedal like a mofo in order to get the car to brake hard. I mean press on the brake pedal hugely harder then I should have had to.

In the first couple laps the brakes behaved normally. If I pressed hard, I’d trigger ABS.

By lap 3 and for the rest of the race, in order to get all the braking g’s I’m used to, I had to press on the brake pedal so damned hard, I thought sure I’d damage something. I’d either bend the brake pedal arm, blow out the MC, or blow something else out. I mean I was really pressing the pedal hard. ABS never triggered when this was happening.

There’s been no changes to my braking system. Same MC, booster, cooling, and pad types (different front/rear) I’ve been using for the last year.

For all the world it seemed like maybe the (front?) pads I had on the car were actually street pads and they were over-heating therefore required massive force to grab the rotors. But if that had been the case I’d have consumed the pads quickly and that didn’t happen. But there’s just no escaping the fact that the best fit for the symptoms is that the front pads weren’t grabbing the rotors very hard. WTF could cause that?

I can’t imagine that it’s the ABS pump. AFAIK ABS pump failure modes are on/off, not a degrading of hydraulic pressure.

Ideas?

#2

Sounds like the vaccum booster is giving out. That is what makes it easier to press. To recreate it, disconnect the booster and plug the vac line to the intake with a bolt then drive forward and try to stop. Just don’t do this test going down a hill toward a brick wall.

#3

I can’t come up with a theory where the booster would work while the brakes were cool, but then crap out once they got hot.

I wish I had a sensor that logged brake line psi.

#4

I had the same thing happen when I used to do a “confidence/sissy tap” approaching the big braking zones. Turned out the check valve was bad in the brake booster vacuum line. New check valve fixed the brake problem then I fixed me.

#5

Pedal pressure is affected by a) air in lines or m/s cylinder problem (soft) and b) vaccum booster (hard). A busted or sticking caliper will have the same pedal pressure but won’t release or engage depending on problem. You will see that by smoking brakes or an oncoming wall. I saw something similar in my girlfriends accord. The vacuum booster was actually applying the brakes as the engine heated and vacuum bulit up. The result was in this case dragging brakes and a quick pedal (i.e. felt like you were slamming on the brakes) but it only really showed up under full load (track time, yes she tracks an accord). Driving down the highway it wasn’t as noticable. Replaced all pads, calipers, rotors, bled everything, no change. Changed booster, problem solved. In your case it actually sounds like the reverse in that the vaccum booster is unable to hold the vaccum needed to assist or there is some other internal issue making it not work. I would just replace it and move on to other tests if that doesnt work but i’ll bet dollars to donuts it does.

#6

But behavior only occurred when the brakes were good and hot. In fact, after a couple slow laps, they’d be back to normal for a couple turns. Then they’d get hot and misbehave again. I can’t come up with a theory that would make the booster, or vac line to booster behave like that.

On the other hand, the idea that it was a single point of failure, like booster or ABS pump, is appealing. If it was just glazed front pads, I should have locked up the rear brakes with the huge pedal pressure.

I really want to put a brake pressure sensor in, and connect it to the Traqmate. Gotta find out outfit that sells a kit.

#7

Good point here to. Forgot about that valve. Maybe start there. The accord didn’t have one.

#8

LOL i think you would put a sensor on your coffee maker if it started acting up.:laugh:

#9

Replace check valve and test…

#10

https://vimeo.com/128651738

The video shows pressure gauges connected to caliper bleed ports. Right gauge is front. Things to note…

  1. At first, rear brake pressure seems to climb a little faster then front. I can’t come up with a theory that explains this.
  1. When the rear pressure hits 350-400psi, the front starts gaining pressure faster then rear. This is likely the action of the brake bias valve. My increasing foot brake pedal pressure went kinda slow during the 300-400psi range so that’s why the rate of psi increase is slow.
  2. Here’s the crazy part. When the front pressure gets to 700psi it quites climbing. I kept increasing the pedal pressure, but only the rear brakes showed the increasing pressure. The fronts stayed at 700psi.

So what the heck is going on that the front pressure doesn’t increase?

#11

Was the 700 psi plateau consistent? Did you try swapping test rigs from front to rear to check God gauge malfunction?

#12

The behavior was consistent. I did not swap the gauges around. That’s a good idea, but if I disconnect a gauge, I’ll have to bleed that corner again and that was a real pita. I guess I’ll have to swap gauges tho to confirm. I’ve wasted lots of time over the years chasing after ghosts caused by a test that wasn’t as conclusive as I thought it was.

I’m thinking that I’m looking at an MC that has found an unusual failure mode. The MC’s hydraulic ram has two seals on it because the ram acts like 2 separate rams, 1 for each chamber. Maybe the chamber for the front brakes has some seal issue such that occurs at ~700psi. Over 700psi the seal shifts and fluid doesn’t compress any more. This is all conjecture, of course.

#13

I put a different gauge on a front caliper and behavior was unchanged. Front brakes still peak and hold at what I’d call a medium brake pressure. Rear brake pressure climbs as it should.

Then I replaced the MC. Behavior still didn’t change. Bummer.

While the MC was off, I went at the 4 lines to the brake bias valve. Since I have an early model, I don’t have the luxury of the simple little 2 line bias valve that sits 12" or so below the MC. I have the complicated bias valve. Using a torch and careful shielding of the various rubber & plastic bits below, to include fuel lines, I torched the shit out of the bias valve brake line connections. With significant tenacity, I got each of the 4 nuts to turn. Which is a good thing, because the bias valve is getting changed next.

#14

Bottom line. It’s not the bias valve. Going to have to torch the brake line nuts loose on the ABS unit next. Bummer.

I used a generic piece of flexible brake line to allow the front hard line to bypass the bias valve. The objective was to see if the bias valve was somehow impacting front brake pressure. The behavior didn’t change. In retrospect, this was pretty predictable. I think the route of the front brake fluid thru the bias valve is a straight circuit. I’ve no idea why the front brake line even goes thru the old style bias valve. So allowing the front circuit to bypass the bias valve predictably didn’t do squat.

Shit.

Gonna be a real bummer when I replace the ABS pump and the behavior doesn’t change.

I’m still wondering if I should be focusing on that clue where the actuating rod that goes thru the booster didn’t seem to be shoved out enough.

#15

One interesting thing I’ve noticed is that the 700psi limit is not consistent. If I “pump up” the MC, I can get up to about 1200psi up front. I can still easily get >1500psi in the rear tho. I don’t have a theory for why the “pump up” thing would give me more pressure up front.

Tonight I got into the peddle box. I checked the specs from Bentley on the booster rod and pedal position, extended the rod a bit, and accomplished nothing. The symptoms didn’t change. I did manage to bust my brake light switch tho.

So I guess ABS pump comes out tomorrow. I’m running out of ideas. I have a bad feeling that I’m not going to figure this out.

#16

Having your pressure gauges installed at the brake calipers makes it difficult to isolate components. Instead of removing your ABS pump, how about hooking up your gauges somewhere between the ABS pump and the bias valve? In essence, you would be examining the bahavior of the only the master cylinder and bias valve combined. If bahavior changes, then add the pump and see what happens. Then add individual calipers one at a time.

The real problem is that we don’t really know what proper behavior looks like. You should test another car. My 1987 is available to help.

#17

I really like the idea of checking another car on which the brakes are good. You are shooting in the dark!

#18

Last night we put my brake pressure gauges on Rich’s S52 e30. The behavior on his car was precisely the reverse of mine. That is to say, front pressure went nice and high, rear pressure stabilized around 900-1100psi and didn’t want to go higher. We were on the low end of that with a single pedal push, but if we pumped up the pedal a bit we got on the high end of that. Obviously, due to the bias valve, this is the kind of behavior one would expect.

I checked a brake pressure chart from the Service Manual and it says that when the fronts are at ~2000psi the rears should be half that. Since my gauges max out at about 2000psi, I’d say that the #'s work out perfectly. Well, for Rich they do, anyways. For me they’re perfectly backwards.

This kind of explains the idea of the lower brake pressure reaching a plateau. I’ve been reluctant to push my gauges or the hoses too hard so I’ve only been testing to 2000-2500psi.

The obvious conclusion is that my brake hydraulics are backwards. Except that I’ve checked that over and over again. Also, I’ve bypassed my bias valve and the behavior didn’t change. If the bias valve is bypassed, there’s no such thing as “backwards.” At least not near as I can figure.

In a couple days some fittings will show up that will let me connect the gauges anywhere in the system. Right now I’m can only connect them to the calipers. So this weekend I’ll connect them after the MC, after the bias valve, torch the damned nuts off the ABS pump, and check it there. This problem is not going to be able to run and hide.

It would be fascinating to find out that my “marketed as” OEM replacement MC that I bought from a local autoparts store has a small rear bore size that’s causing the higher rear pressure. It’s predecessor was exactly the same MC.

I’ve two different new MC’s inbound also. I’m on this like stink.
[attachment=2097]BrakePorpValve.jpg[/attachment]

#19

Fittings came in. I connected my gauges directly to the MC. Brake pressure was nice and high front and rear. That would indicate the problem is likely the ABS pump.

The MC that I’m using, a twin of the one I took back to the autoparts store last week, is marketed as an OEM replacement and having 2 different bore sizes. The fact that the pressure was the same F/R means the MC has the same bore size F/R. So maybe, like Chuck said in another thread, there are indeed MC’s with different bore sizes, but a) this isn’t one of them even tho it’s supposed to be, and b) I guess the only way to tell is measure their pressure. Certainly disassembling them to inspect the piston size isn’t a realistic option. They can be disassembled, it’s the reassembly that is, in my experience, impossible.

I’m really stoked that I’m starting to get results that make sense. The last couple days I was kinda bummin.

#20

I went at the ABS brake lines tonight. I torched off 4 of 5. One of the bastards defeated me and the nut is now rounded. New brake line, a crimping tool, end nuts and F-F fittings have all either arrived or will shortly so I’ll either fab a new line or cut the nut off and splice in a piece of line. With some luck I’ll be swapping in the spare ABS pump tomorrow night. If it doesn’t behave, maybe I’ll fab some bypass pieces to get me thru RA in 2 weeks, then at my leisure I’ll scare up a serviceable ABS unit somewhere.

Hmm. I wonder if I have another ABS unit in my trailer spares?

I have a couple MC’s inbound. Once I realized that I had not a clue what a problem was, I started ordering an assload of parts to make sure I had everything I could possible need right at hand. Once I decided that the MC was not the problem, I thought that maybe I’d test these other MC’s to see if they had a front vs. rear pressure differential. I gotta say tho, I’m a little tired of spending every night with brake fluid up to my shoulders.