Brake Problem

[IsaacMTSU]

The last event was a lot of fun! I have new tires (RE-11s) and new shocks, new rotors, calipers, pads, fluid, etc. but this time I had some trouble with my brake vacuum. It doesn't have enough time to pull a vacuum between brake applications. I try to either be on the throttle or on the brakes with minumal coasting. So after the first brake in the slalom at the top this weekend, I had no power assist on every other application for a half second. It's never done this before, but I have also always had Hawk HP+ pads [edit to clarify: now I don't and maybe I was spoiled by them]. Maybe it takes more travel/vacuum to get the same stopping power that I'm used to.

I tested the vacuum at multiple places and tee'd the gauge inline and on the brake booster and drove around:

* At Idle = 18" vacuum everywhere
* At the Booster = 18" at idle and 24" after a good rev drop

* I let it sit for 2 hours, still 22" at the booster.

The problem is when I brake hard, it drops 12" or so and it only builds about 3"/second. So I cant really get a good brake again until I coast off-throttle for a couple seconds. Which never happens.

If it has 18", on a good brake it drops 12" down to 7" or so. Then it never has a chance to build back up until I brake again and I only have like 7" in there and the pedal is basically rock solid for a second till it catches back up. I found a Hella UP-28 mini pump that can get 15" in 4 seconds for $115. Or, would it be possible/smart to just cap the booster line and run the brakes manually?

[disneyd]

Its not the HP+ pads.

I'm not sure what vehicle you're talking about, but I'd say there is some problem in the hydraulic system or booster. If you google around there are some pretty good howto's on diagnosing booster problems. There are some odd things than can happen to them (like internal leaks, which won't show up on your gauge). Unless you are running a diesel or some sort of crazy cammed V8, you should replenish the vacuum in your booster pretty much instantly when you lift off the throttle. If it's taking seconds to build, there is a leak or restriction somewhere.

[MARKP]

It's the yellow S-10. I agree with Dave. I will also add that if you have messed around with the plug-in location of the vacuum line, you might not be getting as much vacuum as you should in part throttle applications. I haven't ever really looked at your engine but I know SP basically allows anything on vacuum lines so I didn't know if you had messed with any of that.

[IsaacMTSU]

Dave, I edited my post to clarify that I HAD Hawk HP+ pads, but now I don't and maybe I was just spoiled and these new cheaper pads require way more vacuum/travel to get the same stopping power as the HP+. Yeah, I'll have to dig deeper to fins a restriction or something, but there's only 3 vac lines on the engine and all are new.

Mark, it's all stock. The booster line is a 10" long 1/2" ID hose directly to the intake manifold with a check valve. The check valve held pressure well.

[disneyd]

Try this (from the intarwebz):

Code: Select all

 1) Remove vacuum hose from check valve on booster. Place hose from vacuum pump onto
check valve and draw booster to 20” of vacuum.

 2) Let booster sit with vacuum applied for 5 minutes. If vacuum does not stay steady at 20” it is
faulty and needs to be replaced. If vacuum does hold steady at 20” proceed to step 3.

 3) With 20” of vacuum in booster depress brake pedal once and release it. The booster should
transfer some but not the entire vacuum in reserve. Depending on how hard the pedal is
depressed it is normal to see 5-10” of vacuum depleted from reserve. The most important
thing is to ensure the booster does transfer vacuum but does NOT transfer the entire vacuum
in its reserve. If vacuum remains at 20” OR goes to zero the booster is bad and will need to be
replaced. If vacuum transfer is within the above parameter proceed to step 4.

 4) Once again draw booster down to 20” of vacuum. Go inside car and depress brake pedal and
hold down for 30 seconds. You should see the gauge drop slightly and then hold steady.
Vacuum should stay steady as long as you are holding the pedal down. If vacuum drops while
pedal is being held down the booster is faulty and will need to be replaced.

If the booster checks out ok, start looking for reasons why the vacuum isn't being replenished quickly. The check valve may hold vacuum, but is it sticky or restricted in some way that is limiting the build up of vacuum? Can you suck air through it from the engine side quickly/easily?

Also, think about the volume of air your pistons are capable of moving every time they pull an intake charge in.... its a lot compared to the volume of the booster. They should be able to pull a vacuum on the booster in a split second.

[lcoleman]

I have never tested a booster with a vacuum gauge, but your test results make me think it's a bad booster or maybe a sticking/plugged check valve.

Your first post says you have no power assist after the first corner, then later goes on to say it takes more travel to get the same stopping power. Which is it? The former is a booster issue, the latter is probably just poor pads or air in the system. Not many pads have the inital bite at cold temps that HP+ do. If you've gone to something less aggressive, it could very well be that.

[oldmuscle69]

I know you said that you had some new brake parts, but is all the air out of the system and is the master cylinder bypassing when hit hard? And also the ABS system. I know we checked the fuse for it but? If the fuse is pulled does that kill the ABS for a real feel or the same thing? Next step after all that is like Lucas said put the Hawk pads back on. Night and day on my car.

[IsaacMTSU]

Dave, Ill try step 4 and I'll also just get a new check valve. It does go to the floor when I press it really hard for about 10 seconds, is that normal?

Lucas, I have instant power brakes on the first application but after that there's like a whole second of delay after I chop the throttle.

Bob, I did a really good job of bleeding the lines and used Motul 600F. When the booster line is pulled, the pedal is rock solid.

[disneyd]

Dave, Ill try step 4 and I'll also just get a new check valve. It does go to the floor when I press it really hard for about 10 seconds, is that normal?

I don't think so. It might drop a tiny bit, but it shouldn't be stable for 10 seconds and then go to the floor at all if its the first time you hit it after pulling full vac on it. Now, if you sit there and pump it several times without the engine running, you'll drop all the vac out of it.

[IsaacMTSU]

Ok, well that makes it seem like the check valve is letting some air slip by slowly when I hold the pedal. I'll grab one for sure. With the engine idling, I can press it really hard and it will stay pretty hard but goes slowly all the way to the floor. It shouldn't do that? With the engine off (or with the booster disconnected) it is solid all the time and stays put. That means the air is out of the lines and the master cylinder is probably good, right?

[TedV]

The booster on my suburban holds vacuum until you press the pedal. Then it's gets Scarry. Sucking sound and all

[lcoleman]

Usually if it's going to the floor at all that means a hydraulic problem (master cylinder leaking down, air in the system).

[IsaacMTSU]

Yep! Thanks for the help everybody! My lack of power brakes for a second after the throttle chop is not manual brakes that get assisted. I drove it without the booster line hooked up last night and without assist I barely stop the dang thing haha! I got a new check valve, new line, my uncle gave me a nice vacuum reservoir, none of that made any difference. I did step 4 from Dave's post, all is good. The master cylinder has to be letting pressure slip by. There are no external leaks anywhere.

I'll get a new one on there ASAP! Thanks again!

[thrdeye]

This is completely unrelated, but at idle, I could get the Excursion's brake pedal all the way to the floor. JB said that this was normal on those.

[IsaacMTSU]

Chris, after I saw your post I called my uncle. He's ran the Gatlinburg city service center for like 500 years. He said it's normal on a GM, Ford, or some Dodges with ABS. The ABS has a pressure dump that sends fluid into an accumulator instead of the line when a certain pressure is reached. This pressure is seen as "way too much brake pressure for the current speed" and allows the pedal to sink into the floor when at standstill instead of blowing a line. When ABS is activated the fluid is pulsed into this accumulator (really small amount) instead of going to the line. That's its method of ABS control. So, the ONLY way to properly bleed that accumulator when you need to bleed the lines is by using the factory scan tool to cycle the ABS dump valve to fill the accumulator with fluid while you are bleeding. You can do it the redneck way and make the ABS cycle a bunch on some gravel then re-bleed. Might have to do it a few times though to get the air out of the accumulator.

So, I'll try that before I buy anything else!