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I am at a loss. I doubt it is lines since it tracks straight.

I can be moving at 30 mph and stand on it, and it doesn't pull either way.

I have to believe it is the booster.

but since this is lack of power, it is harder to "prove" without that gauge.

And thanks for the web pages. I did find a few online. I was just hoping some national parts chain might carry them so I don't have to wait till next week to figure this out.

but it looks like an online order is at hand. I found one for about that same price with several adapters, so I might go that route.

well yesterday it was crazy bad.
today it is just regular bad.

when I had it inspected last year the guy mentioned that it didn't stop soon enough.
I just figured, heavy car, and all.
I had noticed when I first start out in the car sometimes, but just attributed it to poor vacuum build up after it had been sitting a while.
but I am guessing the booster is going.
wish there was an easier test than getting that test kit. it is about half the price of a new booster.
if I decide to keep going this route.

Two things you seem to be concerned about. 1) you believe the booster is interfering with travel.
2) you believe there may be a master cylinder issue.

To test #1, unbolt the master cylinder from the booster and step on the pedal to see if the pedal will travel all the way to the floor. If the pedal does not reach the floor, your issue is in the booster/pedal assembly. The vacuum assist of the booster is only an assist, meaning there should still be a mechanical connection and you should be able to overpower a defective booster with pressure on the brake pedal. Without a master bolted to the booster, the pedal should push to the floor fairly easily and smoothly. Get an assistant and measure the amount of travel the rod in the booster travels from pedal completely up, to pedal completely on the floor. If the pedal and booster operate correctly, reattach the master, and move on.
2) Now you have to determine if the master cylinder is the cause of your issue. With the master reattached to the booster. Loosen both brake line connections and see if the pedal will still go all the way to the floor. If it does, the issue probably in not in the master. It is possible the master plunger is not completely returning all the way to the back stop of the master cylinder, upon brake pedal release and it is not refilling with enough fluid to reactivate the brakes. If you tighten the lines you loosened for this test while the pedal is still down, you will not pull any air into the system. Once the lines are tight, you can remove the master from the booster to see if the plunger has returned fully to the back stop. I've seen corrosion build up inside the master that will not allow the plunger to return to the back stop and that can cause an intermittent poor brake function. Sometimes the rod between the booster and the master is too long. That will also cause the plunger to not return all the way to the back stop and cause poor brake conditions. If the rod is too short, it will not allow the master cylinder plunger to push enough fluid out of the master to reach full brake pressure (that could cause the pedal to stop before the pedal reaches the floor.) If you suspect the rod may be too long, you can add a couple washers on each stud between the booster and the master and see of that makes a difference in brake quality. If adding the washers does not make an improvement, remove them. The rod between the booster and the master is adjustable, adjust the length in or out 1/4 turn at a time (a little makes a huge difference). If the pedal reaches the floor with the brake lines on the master loosened, and the master's plunger is returning to the back stop, the master is not the problem, and we move on to the proportioning valve.

For a few years, the proportioning valve actually would block off fluid flow to one side of the valve or the other in the event of a line failure. Unfortunately, it would also do the same thing while bleeding the fluid if the bleeding process wasn't done easily. The process to recenter a shifted proportioning valve is not difficult, but requires a gentle touch. To test the valve, loosen either the fitting for the rear brakes, or the fitting for one of the front brakes, and step on the pedal and watch the fluid flow. Tighten the line, and loosen the other line (one line towards the front, and one line towards the rear) and step on the pedal and watch the fluid flow. If either the front or the rear line has little or no fluid flow, the proportioning valve has shifted towards the side with little on no fluid flow. To recenter the proportioning valve, you have to open the side with the fluid flow, and bleed off the fluid until you hear the pop or snap inside the valve, and immediately close off the line, and release pressure. Gently stepping on the pedal should leave the valve centered, and the brakes should be much better.

You have been messing with these brakes too long. Start at the beginning, and work through the process to this point and lets see what you have. Gene

poorboy thanks.
I had not thought about cracking lines open for the master cylinder check.

I did disconnect the master from the booster and verified full motion on the booster.
I am still concerned about lack of assist, but will check the master first now. But with the number of failed univeral boosters, I am not ruling it out just yet even if the master has some issue.

no proportioning valve on this car. was a single pot master, and all I did was run dual lines. (I know, I know, I should add one, just never got around to it).

I think you're running out of piston travel in the MC.
What kind of MC do you have now?

Check the booster by pulling off the vacuumhose, and see if the pedaleffort stays the same.

it is a 70's corvette master.
all the kits at the time only came with those.
I see some of the newer ones do mopar masters, but eh. power brakes is power brakes.

pedal effort is worse without vacuum, but pedal travel seems to be the same. I can't move it any further.

when I get a chance, I will try cracking a bleeder and see if it moves further or not.

I know you said no hydraulic lines were removed, but, to me, the system sure sounds airbound.

Here's and abbreviated procedure, as to how I confirm (or not) in minutes: Plug both master cyl outlet ports. Pedal should be rock hard (or close, with power booster). Not? Bleed master. (Can remain mounted, just slam on some c-shaped tubes).

OK now? Reconnect front fitting (to rear brakes). Still firm? Great. Not, trouble's out back, bleed, excess shoe travel, etc.)

Repeat with rear tube (for front brakes) connected.

The whole process can be done in a few minutes, and, if nothing else, localizes the problem area.

Rick

air in the lines to me would be a mushy pedal, or no pedal at all.
I have a rock hard pedal. I cannot push it past a certain point.
I have verified with the master removed from the booster, I can move it past that point.

now I have to see if opening a bleeder will let me move it past that point.

if not I guess bad master.
if it does, then I am still not sure where to look, but at least I know it isn't the 2 of those items lol.

I checked both calipers move freely. not sure where else it could be bound up. but I will get there once I eliminate the master.

Quote:

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I took off the master cylinder to see if the rod had backed down but it was still adjusted.
pedal has more movement without the master bolted up, so no binding there, nor is it the booster.

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It almost sounds like there isn't enough travel left in the master. Is it possible there is too much preload? Fully retracted the master piston should be very close to the mounting surface (bottomed against the c-clip). Mounting it to the booster should not compress the piston.

Likewise, is the booster fully retracted when the pedal is at its uppermost point?

Had the same problem with my old Toyota Landcruiser. Turned out to be the master cylinder seals (internal piston seals). Only some fluid was pushed down the lines as the brake fluid could circulate to some extend between the two chambers when the brake pedal was applied. New master cylinder solved the problem.

Stanton- I verified the adjustment was correct. you can look into the master to see that there is no piston movement in the master as I pushed it onto the booster.
as for fully retracted, I guess I didn't check that, but my brake light switch is a stop and it doesn't go out further than that. and my brake lights are not on, so it must be coming up to them.


DSG-I had wondered if there was another failure mode for a master cylinder. I have only experienced the fluid bleed by the piston, where you have no pedal. or it slowly falls.

Are you sure that the booster is good? Ive had a booster go bad and it was rock hard pedal. Bad master gave me really mushy pedal. Just because booster is new doesnt mean its good. On my moms Oldsmobile Cutlass Supreme went through about 6 bad masters right out the box before I got a good one. Similar deal once with an alternator as well.

no, I am not sure if the booster is good or not.
it is providing some assist, just not sure how much.
but I figured even if the booster wasn't working well. standing on the pedal, and I mean grabbing the steering wheel pulling myself out of the seat, both feet on the pedal would have locked it up, but it felt like I wasn't moving it any more at all.

so I want to check the master next by cracking a bleeder valve and see if it changes the pedal push distance. I will go from there to see what to do next.

I had the same issue, to a lesser essent, in a car of mine.
Turned out the MC just didn't have enough stroke for the pedalratio in the car. It just felt like the pedal itself was bottomout on something, while you know there should be some more stroke in it.
Swapped in a random MC and the problem pretty much went away. Only need to fine tune the MC's piston diameter for best brake-performance now.

Andrew we need closure!

lol, you are going to need to have some patients then. I will tinker with it again this weekend.
I am looking into the 70's corvette booster now too. see if it will bolt up to the aftermarket bracket.
kill 2 birds as it were.
better booster with a lifetime warranty from a national chain, instead of these flyby night chinese boosters.
depends on what the master cylinder test shows. don't just want to throw parts at this.