Brakes..ugh...help.

[just_ridin]

Put a power booster on a '71 Super Cheyenne...had nothing but trouble since.

Brand new Booster and Master, new rotors and lines...had the issue that the brake pedal wasn't returning all the way back and they were sticking on unless you lifted the pedal back with your foot.

Bled and re-bled everything, adjusted rod length, checked the master cylinder spring...nothing. Returns fine when the engine's not running, but stops about 1 1/2" shy of normal when you fire her up.

Hated to do it but tried the band-aid method of hooking up a spring to the pedal so she returns perfectly...woohoo!! Took her out for a test drive and wow...great stopping truck!! Then 15 minutes in the brakes start locking up again..ugh. Got to the point that they got so hot had to pull over and let her cool down..they were holding so much it was hard to even move....but as soon as she cooled down she worked fine again.

Have seen many, many GM threads where the pedal only comes back partially, but nobody ever seems to post the fix when they find it (or hasn't it been found?).

At wits end here...any suggestions?

[geezer#99]

Did you change your pedal ratio?
Needs to be 4.5 to 1 for power.
Stock is 6 to 1.
You might need to drill another hole about 1 1/2 inches lower on your pedal to attach the brake rod to.

[B. W.]

The brakes are dragging.

Do what geezer said first. After that, check your push rod lengths. Check the one going from the brake pedal to the booster (or bell crank if it a factory style setup) It should line up with your new hole & the pin should fit without you having to move the rod in. A"smidge" is OK but you don't want to "pre-load" the booster. You can adjust the length of the rod (see pic)

Next, check the master cylinder push rod length. This is between the master cylinder & the booster. Don't take the brake lines loose, just loosen the bolts holding the master to the booster. Now, with a helper inside the cab, have him/her hold up on the pedal. You should be able to hold the master cylinder against the booster without having to force it into position. Have the helper slowly apply the brake pedal it should almost immediately begin to move the master cylinder off of the booster. Check this a couple of times, there should be just a little bit of play before the push rod touches the master cylinder. If the rod is not the correct length it should have an adjustment screw on the end.

My guess is one of your push rods is too long. This will cause your brakes to be applied even with your foot off of the pedal.

[just_ridin]

The brakes work perfectly when you're test driving it around the block...no sticking, no lag...they're perfect.

The problem is when you go 10-15 miles and they start to stick after even mild usage...when they cool down it's back to working fine again.

It seems as though the brake fluid is expanding when it heats...which DOT 3 does when it comes in contact even with atmospheric moisture..but it's all new fluid in there and was bled immediately and sealed...that would explain why it's fine when it's cool and locks up when it's hot, but I've never really heard or read anything about that in practical day-to-day use...

[dfwright]

Calipers sticking when hot?

[Wrenchbender Ret]

Yes, the fluid expands when hot. If the pedal rods are too long it doesn't let the port in the master cylinder open to let excess fluid go back into the reservoir.

[just_ridin]

Originally Posted by Wrenchbender Ret

Yes, the fluid expands when hot. If the pedal rods are too long it doesn't let the port in the master cylinder open to let excess fluid go back into the reservoir.

OK...so when putting the booster in, someone here told me to be sure to move the lever higher on the brake pedal to create the proper angle for the rod to get into the cylinder...sounds like that might take away enough space for the rod to not open all the way to let the fluid back in, or am I not processing this right?

Would moving the rod back to the original position be the way to go to see if it doesn't stick, or what should the rod length be (generally)?

[geezer#99]

You moved the rod the wrong direction.
You need to move it down, not up.
You need a hole about 1 1/2 inches lower than the original.

[just_ridin]

Originally Posted by geezer#99

You moved the rod the wrong direction.
You need to move it down, not up.
You need a hole about 1 1/2 inches lower than the original.

I will accept looking like an idiot who did it wrong to get it fixed...thank you very much for the input!

[B. W.]

Don't be so hard on yourself, (We'll do that for you!) No, really, seems like there's not a day goes by I don't learn something the hard way. Any way, issues like yours are what this forum is for. Hope you get it figured out.

[just_ridin]

Originally Posted by B. W.

Don't be so hard on yourself, (We'll do that for you!) No, really, seems like there's not a day goes by I don't learn something the hard way. Any way, issues like yours are what this forum is for. Hope you get it figured out.

Much appreciated...and (hopefully) the final update is that we'll get the pedal adjustment made, and also realized the guy who hooked up that end of things attached the vacuum hose to the carb instead of the intake...so the brakes worked fine on acceleration...(shaking head)...lol

Lots of lessons learned, no doubt. I'm sure I'll be back; next up is a '71 C 20 build...only 48k original miles, taking it back to bone stock as much as we can...what's something like that worth, anyway? After that is the '68 short bed...looking forward to wreaking all kinds of havoc with that.

[randy500]

Post a picture of the pedal under the dash where the booster mounts.
Do what B.W. states in post 3, the push rod that exits the master cylinder should just barely touch the master cylinder when at rest.

Does the pedal return when stuck down if you remove the vacuum line? If so the booster is bad.

There is no way you had the vacuum line hooked up wrong at the carb unless someone used a bunch of adapters to hook up to one of the small vacuum ports like the one to the distributor, most boosters connect to the lower rear of the carb.

Did you change the master cylinder too?
Post a picture of the vacuum connection and booster too.

[randy500]

Did you use the proper shouldered bolt at the pedal? Is it in a bind? It should spin freely if you remove the pedal return spring pressure by slightly pressing the pedal with your hand while trying to spin the bolt, did you lube the bolt where the booster connects to the pedal?
If the booster was out of use for a while the pivots can be sticky, disassemble all the linkage shown in your picture and lube it all up. Its 45 year old linkage and has never been apart for lubrication.

[randy500]

Sorry for multiple posts...been thinking about this more.

Your problem is the pivot points are dry, I just had one sticky like this that was sitting a very long time. When I sold the booster I disassembled the pivot linkage at the back of the booster to make it smaller for shipping.
It wasn't rusty but the pins were very difficult to remove because they were so dry and the zinc coating had that dusty corrosion everywhere. I'm sure that's why the pedal on that truck hung up after application such as yours is doing.

[just_ridin]

Originally Posted by randy500

Post a picture of the pedal under the dash where the booster mounts.
Do what B.W. states in post 3, the push rod that exits the master cylinder should just barely touch the master cylinder when at rest.

Does the pedal return when stuck down if you remove the vacuum line? If so the booster is bad.

There is no way you had the vacuum line hooked up wrong at the carb unless someone used a bunch of adapters to hook up to one of the small vacuum ports like the one to the distributor, most boosters connect to the lower rear of the carb.

Did you change the master cylinder too?
Post a picture of the vacuum connection and booster too.

I appreciate all the thought and input Randy...thank you.

The booster, master cylinder, calipers, and lines are new here...not sure if there could be a sticky pivot point if the booster is brand new?

Spoke with an uncle who's done nothing but work on cars and trucks for the last 50 years...told him about the problem and he didn't hesitate a second in asking where the vacuum line was connected. You're correct, we had it at the lower rear base of the carb. His point was that vacuum will be created upon acceleration, but not during braking...which accounts for the fact that after you brake it stays down. Hoping he's right in that changing it to the intake will create more constant vacuum and return the pedal.

I'm ready to move the lever position back down on the brake pedal to make things less complicated, and I'll take a look at the pivot points and make sure they're properly lubricated before we get her out for a test drive.

This thing has taken us so many ways I'm about ready to start pulling hair/drinking battery acid...will keep you posted if you'd like after we get the tests done after work tonight.

Again, very much appreciate the input! Hopefully I'll be able to be as helpful on here as you've been.

[randy500]

Vacuum goes away during acceleration since the throttle valves are open, it returns as they are closed. Unless you are braking while accelerating you will always have more vacuum while braking. The vacuum connection is not your problem.
Since your booster is new and the tolerances on those pivots on the reproductions are so loose that is not your problem either.
I thought the one in the picture was the one you installed.
Post the pictures though so we can be sure.

[just_ridin]

Originally Posted by randy500

Vacuum goes away during acceleration since the throttle valves are open, it returns as they are closed. Unless you are braking while accelerating you will always have more vacuum while braking. The vacuum connection is not your problem.
Since your booster is new and the tolerances on those pivots on the reproductions are so loose that is not your problem either.
I thought the one in the picture was the one you installed.
Post the pictures though so we can be sure.

I'll have to take them tonight...the shop isn't at my place.

[Wrenchbender Ret]

Check the vacuum source. If it has no vacuum at idle but has vacuum at part throttle then it is hooked up to a ported vacuum source. Like Randy says someone must have adapted it down as the power brake uses a 3/8 outlet. Depending on the carb make, there can be a nipple on the base of the carb or a pipe plug you can remove and put a fitting in. V-8 engines have an outlet on the manifold in back of the carb.

[just_ridin]

Aaaaaaand it's fixed & working like a champ.

A combination of the two...changing the vacuum to the manifold got the pedal snapping back in place, and adjusting the rod stopped the locking up.

HUGE sigh of relief...spent way too many hours trying to get that thing fixed!

Now on to the '71 3/4 ton...

Thanks again for all the input..huge help.

[randy500]

So which rod needed adjusting the master or the pedal?

[just_ridin]

Originally Posted by randy500

So which rod needed adjusting the master or the pedal?

Moved the pedal rod back to the original (lower) position and shortened by 3/8".