Too much travel before brake booster kicks in?

[JLeather]

My '79 has too much brake pedal travel before it engages the booster. I thought it was the brakes out of adjustment but I checked and adjusted them (the rears anyway) and it didn't change anything. The pedal travels a good 3 inches with essentially no resistance at all before it engages the booster (can hear the booster sort of whoosh a little when it finally hits). After that it firms up pretty quick and the truck stops. Not sure if there's an adjustment I'm missing out on between the pedal and the vacuum booster, or if this is a sign of some sort of problem with the brakes I haven't properly diagnosed yet?

[cadillac_al]

It sounds like the master cylinder is going out unless there is some air issue from running out of fluid or brake line replacement.

[cleansquare]

I'm having the exact same issue. As far as I know the booster is not adjustable, at least not the one I bought.

My brake light has been on in my 85 for about 6 months. I didn't know it was indicative of q bigger problem, thought it was something electrical.

Finally checked the master cylinder, the front brakes were full but the rears were bone dry. Filled them up, bled them and eventually got fluid but no pressure.

Couldnt tell anywhere that the fluid could have gone. NOTHING was leaking, or wet. I bet that it had long since dried up.

Replaced the master cylinder and bench bled it, no change. While in the process figired out my old one had leaked and that's where the fluid went. Changed the booster, no change. Brake light on amd still no pressure to back.

Heard about the front combo valve. Also the rear proportioning valve. Removed rear and cleaned it, no change. Read up on the front one, apparently it has a valve that whej one system fails (rwar in my case) it blocks off that system and allows you to use the frobt still. So I've been running on front brakes only for months.

Tried all the methods to reset it, nothing worked so I removed it, ordered another one from online and got it in last night. Brake light went off, pressure came back to the rear, truck stops better, but still a soft pedal. Does like you are describing.

Bled the brakes till I'm blue in the face. No air...mc, boostet and prop valve all new. Adjusted rear brakes. Dont know what else to do. Stops better now, but still a soft pedal!

[JLeather]

That's weird that you mention the rear brakes not working for a while. When I bought this truck (recently) the previous owners told me they'd had the rear brakes rebuilt. Well the guy put them back together wrong and didn't properly seat the pushrods from the wheel cylinders in the brake shoes so they overextended and didn't move the rear brakes at all. I fixed all that, but the pedal feel didn't change. It could well be this front combo valve you're talking about. Got any links that talk about how to reset that? This truck does have front discs, but do you know if they had that valve in '79?

[cleansquare]

Dunno if 79 had it...sure it did. Follow the two lines from the mc down around below the fan shroud on drivers side. There are ways to reset..like the small pin facing the frame on the combo valve that you are supposed to push in and reset...this did not work for me. Also, fill mc up and take truck on road and try hitting the brakes hard in forward and reverse. That's supposed to be another way which did not work. Thrn try to open the bleeders for the front wheels, close the rears. Pump the brakes. This will in theory create pressjre that makes the valve pop free again. Did not worn for me.

I had to remove and replace mine. Cant get them at parts houses.

Here's some pics.











The reset pin is under that boot on the last pic.

Ypu should have 2 lines entering the prop valve feom the mc. In inlet for the rear and inlet for the feont. Then, two small outlets goingnto the front two wheels, and a large outlet going to the rear which splits off later down the line.

The electrical connector is to trip your red brake light when the valve is off center.

Your valve could be centered but have you bled the rears? Any air come out? Any pressure?

[JLeather]

Yep, I got one there. Same place too, under the driver's side corner of the radiator. I'll try the pin this afternoon. Anything special you're supposed to do as part of the "reset" or just push it in and then drive around a bit?

I didn't bleed the rears because the m/c was full and nothing leaked (plus I was working alone yesterday). If I get time today I'll try that, they're calling for 8-12" of snow tonight so if I don't get it done tonight it might be a while before I feel like laying under the truck again.

I did do the usually forward/reverse hard braking mostly to see if the brakes would adjust themselves a bit tighter than I did manually.

[cleansquare]

Well pushing the button did not work on mine. So I had to replace. I'm inyetesyed to see if you have pressure at the rear or just a trickle. No brake light on? Is it connected at the valve?

[LONGHAIR]

If you don't get the pressure "balanced" by getting the rear bled, anything you do to get the valve centered is worthless. It will just pop again.

If you are sure that the rear is correct and has no leaks, loosen a front bleeder, push the pedal down, to the floor. At the floor, it should "click" and travel a bit more. This is the valve re-centering. Tighten the bleeder, let the pedal back up and let it sit for a few minutes.
Check the fluid level in the master cylinder, fill if needed.
Now open ALL of the bleeders and let them trickle. Keep an eye in the fill level of the master cylinder, do not let it get low. Let it flow for a while, this will flush out some old fluid and any air left in the system. This is known as gravity bleeding and it works very well, plus it keeps everything balanced....not re-popping the valve.

ETA:
To address the original question.
Yes, there is an adjustment. The link rod that goes from the pedal into the back of the master cylinder has a threaded adjustment to it. Just be sure that you are "just taking up the slack", you don"t want to actually put pressure against the back of the plunger.
If this doesn't fix it, you probably have a master cylinder problem (internal leak)

[JLeather]

Ok, not quite sure where to go (although I have my suspicions). It warmed up enough to get some work done on the truck. My buddy came over and attempted to bleed the brakes with me. Started at the right-rear wheel...nothing. A couple drips from gravity, but not an ounce of pressure. Not the biggest surprise ever, given the poor rear brake performance. Went to the right-front...nothing. That was more surprising, given that I drove this truck 30 miles home when I bought it, at speeds up to 55mph. Kept trying the front and started getting huge air bubbles, but no fluid. The more we tried bleeding it the softer the pedal got. Tried the left-front...nothing. Drips, no pressure. Pulled the lines off the m/c and tried bench bleeding it. Plenty of fluid, no issues bench-bleeding. Put the lines back on the m/c and found that suddenly we had plenty of pedal pressure. Started with the front-left and got a nice healthy squirt of clean fluid. Tried it again, got less fluid. Tried it a few more times and back to nothing. Pedal got softer each go-round with the bleeding.

So, I'm suspecting the m/c is bad, but I can't figure out what in it has failed that it will build less pressure each time it's bled. Can't even figure out if the proportioning valve is bad until I've got good m/c pressure. No leaks, system was (and continues to be) full of fluid. So where did all the air come from as we tried bleeding it? It's almost like the more we bleed it the more air that shows up in the system. Like the m/c is sucking in air somewhere, but isn't leaking?

[Oldriginal86]

Could be the pushrod is too long. This would not allow the bore/piston portion to fill with fluid when the pedal is released. Is the pedal coming all the way up?

[JLeather]

The pedal is coming up eventually, but with no brake pressure it's pretty slow. I didn't take the m/c off the truck to bench bleed it, just disconnected the steel lines and cycled fluid around using the truck pedal. It worked fine when I wasn't asking it for any pressure, nice big healthy squirts of fluid every time you depress the pedal, and after that when I hooked up the steel lines again I had great brake pressure for one good pump. After that it had less pressure each time I tried pumping them up to bleed until finally it was back to none (and I do mean none).

A rebuilt m/c was only $28 at my local parts place. At that price I'm just gonna replace it. I can get some help from my buddy again on Saturday and the weather should be nice. We'll see what happens...

[Houston Ben]

I'm having a similar problem, I had the back ones working for a day and back to crap, looking forward to your results.

[motornut]

stock brakes?

"Started at the right-rear wheel...nothing. A couple drips from gravity, but not an ounce of pressure. Not the biggest surprise ever, given the poor rear brake performance. Went to the right-front...nothing"

should be farthest from driver to closest so RR/LR/RF/LF
I've had a MC front part got low,light went on,refilled MC
bled as above,and it went out

[JLeather]

Yep, bone stock brakes. 13" drums in the rear and whatever the HD discs are up front.

Got the new m/c in and the results are mixed and somewhat disappointing. As soon as I put it in I got fluid to the back brakes, so that does indicate that my old m/c wasn't generating any pressure to the rears. The rears bled nicely, starting at RR and then LR. At that point the pedal was pretty firm at about 2" of travel. Big improvement. Went up to the fronts and bled out a lot of air, not surprising since before the m/c acted like it was pumping air in with every stroke. The pedal felt nice and firm, no more air was coming out of the brakes, etc.

Here's where it gets disappointing. As soon as I started the truck the pedal went back to being extremely squishy, travelling almost to the floor. The truck stops again at least, but it takes a solid 5"+ of pedal travel before the brakes even start to drag, and it's almost on the floor when the truck stops. I'm going to let it settle and try bleeding the brakes again tomorrow. I can move the truck around, but not comfortable enough to try driving it.

[motornut]

Originally Posted by JLeather

Yep, bone stock brakes. 13" drums in the rear and whatever the HD discs are up front.

Got the new m/c in and the results are mixed and somewhat disappointing. As soon as I put it in I got fluid to the back brakes, so that does indicate that my old m/c wasn't generating any pressure to the rears. The rears bled nicely, starting at RR and then LR. At that point the pedal was pretty firm at about 2" of travel. Big improvement. Went up to the fronts and bled out a lot of air, not surprising since before the m/c acted like it was pumping air in with every stroke. The pedal felt nice and firm, no more air was coming out of the brakes, etc.

Here's where it gets disappointing. As soon as I started the truck the pedal went back to being extremely squishy, travelling almost to the floor. The truck stops again at least, but it takes a solid 5"+ of pedal travel before the brakes even start to drag, and it's almost on the floor when the truck stops. I'm going to let it settle and try bleeding the brakes again tomorrow. I can move the truck around, but not comfortable enough to try driving it.

Your brakes may need to seat
Or the rears need adjustment
try the e brake ,they are different systems, but if that doesn't work , it still needs adjustment
If the adjusters etc are working, backing up and hard stops can adjust it.

[JLeather]

I adjusted the rears by hand until they just started to drag and then backed them off a click. I tried hitting them hard in reverse, didn't seem to affect anything. The star wheels, springs, and linkages in the rear are all new and seem to be working. If it was a brake adjustment issue shouldn't I have the same problem when the truck isn't running?

I went back out and checked the brakes, and with the truck not running the pedal felt pretty firm and only had 1-2" of travel. Started the truck up, the pedal went to the floor again. A friend of mine said he had a truck do something similar and it turned out to be the brake booster, although I don't see how that could be my issue? I can put a vacuum gauge on the booster and check for leaks, is there any other way they can go bad?

[cadillac_al]

For some reason I never have good luck bleeding brakes with the vehicle not running so I always bleed brakes with the engine running. With a new master and combination valve it must be a bleeding issue. If the booster wasn't working, the pedal would be very stiff like when the vehicle isn't running.

[JLeather]

Ok, poked at the truck some more today. I have a suspicion that my "new" (rebuilt) m/c might be bad. With the truck not running when I step on the pedal it firms up, but if I keep pressing on it the pedal creeps towards the floor. Also, I noticed that when I do that do it (i.e. force it to keep moving after it firms up) when I release the pedal a ton of bubbles appear in the rear section of the m/c. I did this probably 25 times and every time I got a ton of bubbles, but the level of fluid never went down so it's not leaking. Where could all this air be coming from? This seems similar to the issue I had with the previous m/c. Not leaking, but somehow creating air. I'm going to vacuum-bleed the truck and make certain there's no air in the lines, and also re-do the bench bleeding on the m/c and after that if I'm still getting all these bubbles in the reservoir like this it must be the m/c. Has anyone else seen this before?

I checked the booster with a vaccum gauge and it pumped down and held vacuum without loss for a good 10 or 15 minutes, so that doesn't seem to be my issue.

Just to clarify, I have not replaced my proportioning valve because I was able to get fluid at all 4 wheels with the new m/c. Any way that could be allowing air into the system?

[solidaxel]

Following!

[E.TexasTwister]

I'm following also. What has happened?

[JLeather]

Sorry it's been so long. I had a couple other projects take precedence over the truck. Long story short, my "new" m/c was bad. I put another new m/c in and bled the brakes and everything is working well now. They're still "typical chevy" spongy, but they work and stop the truck great.

[t_graham11]

I've got the same problem on my 82 but not my 83, some good info in here thanks fellas