hmmwv15

Pedal is soft, like there is no pressure at all. When you pump them really fast, it seems like it wants to build pressure, but just remains soft. When we got the truck we replaced the wheel cylinders, I rebuilt the calipers, and replaced the master cylinder. I have just finished installing another master cylinder and the problem persists. I took off the brake booster and depressed the "plunger" and it seems to be fine. Is there a possibility that it may not be building enough vacuum in the booster? Yes I bled the brakes correctly, don't ask me that. I'm ready to take it into a shop to get them done, because quite frankly, I do not want to mess with them any more. It's on an 85 4runner. Thanks, Sean

4Crawler

iTrader: (3)

You could put a vacuum gauge on the intake and see what the vacuum reads. Make sure the vacuum hose connected to the booster is intact, open inside and not collapsing under vacuum. Any change in brake pedal feel with the vacuum connected to the booster and not?

hmmwv15

Well, I don't have a vacuum gauge, but I did take the line off of the brake booster, and the brakes feel fine without it...

(this is sitting on jackstands in my driveway, I didn't actually go out and test drive it.)

hmmwv15

Well, test drove it first with the line attached, and when gently applying brake pressure gradually (takes no effort to push the pedal to the floor), it gets to a point where it acts like I have slammed on the brakes. they stop me, but obviously I still don't trust them. Took the vac line off and drove it, and I now understand exactly how much of an important role the booster plays. Yeah it stopped, but it took a heck of a long time. Anyway, I'm going to limp it into auto shop so the auto teacher can see what it's doing.

For now, I'm either thinking a faulty master cylinder or faulty brake booster.

bigt

did you bleed lspv mine did that UNTIL i bleed that darn thing and it was so rusty took me a while lots of heat and thrust to crack it but when i did air bubble city.. i knew i was in buisness

hmmwv15

yes, I bled it. No air.

GSGALLANT

If you get good pedal near the bottom (i.e. it stops your truck well, but at the very bottom of the pedal stroke) it sounds like your rear brakes are out of adjustment. Make sure your rears are adjusted properly first (do a search on here if you're not sure how to adjust them).

Also, did you bench bleed the master cylinder before putting it on? The chances of getting all the air out of the MC if you don't bench bleed it before installing it are slim.

hmmwv15

Bench bleed the master cylinder???


yes, I bench bled it. Rear brakes are adjusted to where the drum drags on the shoes, so they are in adjustment. This is why I'm baffled. I have done every adjustment I can think of. Only thing I haven't messed with is the booster. Could it be that the pedal is out of adjustment? (wouldn't make sense if it were working fine a few weeks ago) but maybe the little pushrod coming out of the booster, or where the other rod out of the booster connects to the pedal? It is a master cylinder out of a 86-89 model 4runner, could it be that the 84/5 models had a different distance between the master cyl and the booster pushrod?

hmmwv15

Just throwing this out there, I have a newer 22re from a 93 or 95 toyota pickup in my 85 4runner. (courtesy of po) Could it be that a newer engine has a different vacuum rating for the booster than the old truck? The brakes have always been a little finicky since I got the truck.

904_runner

possible. call up a parts store and see if there is a difference in part numbers.

hmmwv15

There should be a different part number, because most of the time they are shaped differently. I'll call and see if they can give any specs though. Good idea.

hmmwv15

Well, I thouroughly bled the brakes again, and it helped the pedal significantly. It now goes about 1/2 way down and stops.

I can slam on the brakes and lock them up if I try hard enough (if I hold my foot down, the pedal will slowly sink to 1/2 way). I get great braking power when forced, but when I ease on the brakes the pedal just sinks with no effort what so ever 1/2 way down and stops firmly, giving me some good breaking power at the 1/2 way point, but there is no "build" to the power, it just all the sudden wants to allply most all braking power when eased onto. If you barely ease onto it, I can get them to gradually apply. I just can't figure it out.

Basically, I'm getting great braking power, so there can't be any more air in the system, (right?) the rear brake shoes are adjusted to drag on the drums, booster is working, (as far as I can tell) and the wheel cylinders don't show evidence of leakage (so far) so what does that leave out? LSPV?? But how could it be that? What would cause the brake pedal to sink, but retain braking power? Could it be that the wheel cylinders (one or the other) has a busted internal seal somehow? I'm stumped!

FredTJ

It's unlikely the booster in and of itself.
The booster is the "power" in the term "power brakes".
If the booster fails, you'll still have brakes, however you'll need both feet and then some to stop it




Fred

hmmwv15

yeah I have basically narrowed it down a bit:

1. some how there could be more air in the system, even after thouroughly bleeding the system a few times.

2. something wrong with the calipers, wether it's air in them, or that the calipers are still sticky, even after the rebuild a while ago.

OR, there's some mythical creature inside the brake lines that drinks brake fluid and excretes air.

I will hopefully have a buddy coming over to help me look at them this week. If not, as long as I'm careful I can still drive it around.

hmmwv15

Possible that a wheel cylinder can leak internally w/o showing any fluid dripping on the outside? Even though they are practically new....

Or the LSPV is shot? Since you get the pedal play from the rear and the braking power from the front, it makes sense that something in the rear is busted. I get great braking power, just no pedal.

Just a thought, will probably go ahead and replace both wheel cylinders in a few days, and possibly bypass the lspv or something.

lham

I've seen the rubber lines in the system not let fluid thru after age. I replaced with new and no mo problem.

hmmwv15

got new extended steel braided, so definitely not the lines.

jusfloorit

Hey did you ever figure the problem out?

I basically have the same problem. I also replaced the master cylinder thinking it was that. The brakes were better but still have the same "sinking feeling".

Was gonna post up about it but used the search button first.

Was also thinking it might be the LSPV but wanted to see if anyone else had this problem and how they fixed it.

Thanks.

hmmwv15

After 14 hard hours of wrenching in the cold, sleet and snow, it turned out to be the booster. The used one I had replaced it with originally turned out to be bad. Hope that helps anyone in the future, if you have any questions I'd be more than happy to help.