Brake Booster Tech

Hey guys, I need to consult the brain trust on what I think is a booster problem.

73 Barracuda Power disk/drum, adj prop valve.

I have been completely through the brake system, new rotors, drums, shoes, pads, rebuilt calipers, new style wheel cylinders with residual valves, 1 1/32 AL MC listed for a Gran Fury, parts store reman booster, PR clearance set at .020" using a tool.

The hydraulics are fine, properly bled and functioning.

So, my factory brake booster died, but before it did, the pedal got very soft and would travel about half way before any meaningful braking started, and then I let off the pedal and I have immediate braking when pushing the pedal. Then the booster failed completely.

Parts store reman booster, same pedal behavior. I don't doubt that my rebuilt booster could be a POS, but my inner A&P loathes throwing parts as a troubleshooting measure.

I have tested the booster using the FSM method and methods I found on the interwebs. It seems to pass those tests, but I dunno.

Can anyone put a finger on what would cause these symptoms? I am befuddled. Faulty booster is where I'm at...

Thanks

If there is no vacuum loss and provides boost I think the booster can be ruled out. It sounds like there could be air in the lines. With engine off the booster should have a vacuum reserve. After one or two applications of the pedal that should have bled off. Pedal at this point should be rock hard provided there are no leaks and no air in the system.

Yep, when the booster bleeds its reserve, nice high, hard pedal. Also removing vac nets a super hard pedal, but no need to let off and take another "bite" (if you will).

On the air in the hydraulics, that was my first check on the old and reman boosters. I've pulled a couple of quarts of fluid through at all four corners using vacuum pump. I am confident of no air in the lines or MC. Nice solid jets of fluid. With air in the lines, multiple pumps gives you better performance. In my situation, letting off the brake after application nets a high hard pedal for the next push, with no improvement in subsequent pushes.

It is the stock style booster for a 73 E body and it feels to me like the volume of the booster might be mismatched to pedal travel/MC stroke?? Its like the booster needs a re-charge after initial application. Could a small internal diaphragm leak cause this?

I will add that in reverse, I get a nice initial bite, going fwd, mush pedal with a Flintstone's level initial bite. I have watched the actuation at all four corners and I can feel braking happening when I touch the pedal, just not much.

I probably should have stated this in the OP, but it is outside my experience and seems unlikely to me. I have overheated my pads to the point of smoking two times in all of this. On the initial bed in I was having so much fun getting from 20-60 that I did it until things heated up and when I came to a stop, brake stink and white smoke. Did the same a few weeks later while adjusting the prop valve. In the pre-overhaul setup with the same symptoms, I had organics front and rear that had been seriously abused and installed were installed without dressing the drums/rotors.

The pads are EBC reds, which I've loved on several other cars, but could I have smoked my pads? They didn't appear to need it, but I roughed the pads up on a flat plate with 180 and blew them clean, I also used some 120 to scuff the rotors. No change. Also, performance doesn't change when the brakes are cold or hot (unless heated until they go away).

I'm with you on ruling out the booster 469, but I can't think of anywhere else to troubleshoot. And, that said, my intellectual curiosity has failed me when it comes to more than just high lever brake booster theory.

Booster shouldn't produce a soft low pedal. It sounds to me like an internal leak in one of the master cylinder circuits. Pinch the flex lines and see if the pedal is hard? Still soft? I'd take it apart and have a look at the insides of the master cylinder. Seen a few off the shelf units that were rusty internally.
Doug

I agree with dvw.
I used to be able to feather my pedal and cause it to go to the floor because the piston had a cup seal that would bend backwards and allow fluid to bypass.

It could be poorly adjusted rears or missing residual valve for the drum, but I am inclined to go with master.

But lets walk through this, as I too had throwing parts.

Bad booster can fail a few ways.
1. internal check valve goes bad. Pulls brakes to the floor while driving.
2. no longer has vacuum assist and pedal is hard to push
3. internal check valve is sticky causes rapid braking instead of gentle braking as expected.

None of those you describe.

Master cylinder failure
internal bypass soft pedal, to the floor, min braking. Could be intermittent
or firm pedal that bleeds down.

does not fully release the brakes - this could also be bad adjustment of the push rod to the master.

leak in the piston
also soft pedal or firm pedal but it bleeds down as it is being held
oil in the booster.

booster tests, could be to remove vacuum and see if pedal effort is the same. Confirming vacuum leak or no boost.

master cylinder test as mentioned to clamp the soft lines, but really the best choice is to put plugs in the master and see if the pedal effort changes.
if it still does that quick pump down and the hard pedal, you know it isn't a brake adjustment.
if it is rock hard and doesn't move then it is further down stream.

at that point I would verify the residual valve is there and then check all the soft lines for ballooning as well as the drum brake adjustments.

also it is possible the disc could have slack that needs taken up if you have wheel bearing issues. going bad or not tightend back up where the disc wobbles as you drive pushing the pads out.

Thank AndrewH and dvw. I'll plug the MC and do that check.

I'm going to move on from the booster at this point.

On the residual valves (if the MC checks out), I have new manufacture units that theoretically have the residual valves built in along with a late model MC, with no residual valve. How can I check that this is the case?

The back brakes don't seem to be a problem, as in reverse (at reverse speeds) I have better braking than going fwd. This would indicate that an MC problem would be in the front circuit, correct?

This guy is hard to listen to, and may be a bit crazy, but there is some good information if one can stand to watch. https://www.youtube.com/watch?v=oqq7WC-VamM

As said, cap the MC with with 2 plugs from the Edelman cabinet at your parts house & with it idling, if the MC is (1) good (try easy pedal & stomping) (2) bled out (3) pushrod clearance OK (4) booster and or check valve OK: then the pedal should be rock hard with virtually no travel.

residual valve you would have to look and see if it is there.
it would be in the rear port of the master.
if that is the stock master for the car and you swapped to front disc, my guess is they are missing.
if you have swapped the master before, most parts houses don't include them.

a test I guess would be to see how far the shoes retract in the drum.

for me when I adjust the drums, I lock them up with the spoon then back them off till they just barely move.

the pumping up working makes me question the residual valve and poor adjusted rear's.

you would need volume to move the rear wheel cylinders out enough before the shoes hit the drum. Then your brakes would be fine.
till the shoes retracted a bit then you would be doing that again.

Thanks 469, I'll endure the vid for it's gems in my garage.

Robert, I'm going to do the MC check 1st (I plan to again bleed it before capping the ports and testing, just to be sure).

Andrew, I'll move on to checking the rears if the MC checks out. I adjusted to "light drag" as opposed to "barely move". Got it on watching the shoes retract. That notion has me thinking about some noise I get from the rear when braking.

As far as my hydraulics, the MC is a 1 1/32 unit listed for 89 Dip/GFury with no residual valve. The new build wheel cylinders theoretically do have residual valves. This is based on Ehrenberg articles.

Thanks guys, I should have some time tomorrow PM to dig into the MC.

Chris

Gents, by way of an update, I re-bled and tested the MC. Passed. I checked shoe movement on retract, does not appear excessive. Tightened up rear adjustment. No change.

Yet another Vac pump bleed. No change. Traditional pedal bleed, no change.

It has to be air in the system....

Did an overnight gravity bleed on the rears, and the fronts are bleeding today. I'll let you know.

One other thing I touched on, but I had been running an old MP adj prop valve on the rear output side of the funky 73 prop valve with the shuttle valve in it. I replaced that mess with one of the Willwood adj prop valve/distribution block combo units. It gives far less adjustment on a per-turn basis than what I had. I have it set to max rear flow for the gravity bleed.

Also, to install that wilwood valve and my big exhaust, I had to get a couple of 10" prefab brake lines and bend them to my will. I checked and verified the integrity of the bends, but I theorize given my routing, they could potentially be an air trap. Hopefully the gravity bleed will deal with that, if it is the case.

Sure would be nice to have access to a hydraulic brake mule like I had when I worked in hangars.

what about capping the forward MC port with a plug & see if it is now high & tight useing the next to the firewall port for the fronts. this would elim everything rearward & you'd be dealing with the easier to deal with (shorter lines/no prop valve) front brakes (tho discs can be hard to bleed out).

Great Idea, thanks Robert, I'll add that to my troubleshooting chicken scratch notebook. Isolate the circuits like electrical troubleshooting.


That said, success! My confidence in my bleed was misplaced. A 12hr gravity bleed on the rear, then one on the front took care of it. High, hard pedal. I do have some Popeye arms now from unsuccessful vac pump bleeding, and the traditional pedal bleed didn't get it either. Gravity is your friend.

A tip to pass along, the Bosch 3,4,5.1 high temp fluid gives much improved brake "feel" and doesn't hemorrhage out the MC cap vents when you make the fronts go away with extreme use.

Thanks all, I learned a lot going down rabbit holes troubleshooting a basic problem. Once again the KISS principle proves its self once again.

You sound pumped (enjoy it while it lasts!)

Not to question your job.. But you didn't put the calipers on the wrong side, which put the bleeders on the bottom of the caliper and traping the air in the caliper? I've seen this done before...

Thank you for posting an update. It allows all of us here to learn and doesn't leave anyone wondering if it was able to be sorted out. This is how this forum becomes so valuable.

Pumped, I see what you did there Robert

Negative on the calipers on the wrong side John, and no offense taken. All thoughts and advise welcome and wanted. And, I have done that before.

469, EXACTLY! It is highly frustrating to spend time searching and find a thread on point, with good info and the OP leaves you hanging.

There outta be a sticky at the top requesting that people post their resolution instead of taking the money and running. It's just good form.

Yeah but it took you awhile!