searcher

Been there done that. Just an idea but I had almost the same problem with my '89. I could apply the brakes and stop but if I needed to stop quick then it was like no power brakes and there had better not be anyone close in front of me. I checked my vacuum and it showed 20 inches but the needle fluctuated. I tried everything with the brakes from bleeding to adjusting to changing pads and nothing helped. I checked the vacuum at each pump (there are 2) and sure thing one was working solid and the other was causing the problem. I blocked off the bad one and problem solved. You may have a bad booster but normally just see if you can shut the engine off and still have booster vacuum for 2 or 3 pumps. I think the sensor you are refering to is the one that monitors vacuum and turns on the brake and antilock lights on the conv. center. Is it on the left fender close to the hood hinge. Good luck and let me know what you find.

edwinsmith

Some of the confusion may be from the fact that the vacuum booster itself stores vacuum for stopping. If you had vacuum of 15 (21?) inches of mercury at one time the higher vacuum should be stored in the booster ON BOTH SIDES OF THE DIAPHRAM. There is a check valve that prevents it from leaking back to the lower level should your engine stop and your other vacuum stuff drained the remaining vacuum in the lines.

Unfortunately there is no way to check if the vacuum stored in the booster is bleeding off without your foot on the pedal.

HMMMM! That gives me a thought. If the rod from the pedal to the booster were a bit to long or the pedal is being held down some by something under the dash it might be bleeding your vacuum off continuously. Is there a pedal return spring that may be broken?

Since you're swapping out the booster anyway if you still have problems it might be something to check.

There is another valve on the piston rod from the pedal which then lets outside air into the back side of the diaphram which assists pushing the piston of the master cylinder down. When you back off the pedal however some air is let into the front side of the diaphram which equalizes the pressures somewhat and backs off the pressure on the MC piston.

This is why you only get one or two assisted stops with no NEW vacuum supply.

Edwin

flashgordon

I agree, make shore that the rod is not aplying pressure on the booster when your installing the master syl back to the new brake boost.

Micaiahfied

that's an interesting thought... i am actually having a problem with the brake lights staying on.... i just adjusted the little thing closer but it doesn't seem to come back as far as it did... isn't the sping in the MS what causes the booster to come all the way back? when i disconnect the rod to the booster the pedal just swings.

Micaiahfied

searcher, i was wondering about that as well. if i start the truck with the vac hose off it makes almost a tick tick tick tick sound, and it doesn't pull constanly like my 92.

searcher

When my lower pump was failing I seemed to have good brakes at 40 mph or less but if I had to stop above 40 mph the truck would start slowing and then the pedel would get hard and no power assist. This truck is the hardest I have ever seen to stop with out power brakes. Just an idea, I think that the fact that the check valve at the booster is mechanical and since we have two pumps The good pump is opening the valve and the bad pump is causing the vacuum to dump. This had me really stumped because the booster seemed to pass all the tests in the manual ie: pressing pedel without engine running, number of pumps with engine off, and applying pedal and starting engine. I checked vacuum at both pumps and ditermined that the lower pump was causing the needle to fluctuate rapidly whereas the upper pump was steady at 20 inches or better. I removed the hose from the lower pump and blocked it with an old sparkplug and put short hose on the bad pump output and blocked it as well (just to keep trash out). The brakes worked perfectly. Now the question is If the vac pumps can't replace each other and keep the brakes working if one fails, then why do we need two high dollar pumps on our engines. I have replaced my bad one now. Good luck.

flashgordon

K it been a long time sence i have had a master syl. of so i may be heading you up that creek w/ or w/out a paddle

Is'nt there a adj rod that gos between the master syl and the brake booster? if there is and you have too tighten the bolts of the master syl to get the rod into the brake booster......(usally this problem is nottis by the caliper sticking when it gets hot or the brakes start to drag from a start.) you may have found you problem....just a nether angle to look at
If you haven't installed the new booster yet,this might be a good thing to check! the rod should'nt be loose when master syl is installed but pressure before the master syl mates the brake booster is'nt good eather.

Micaiahfied

searcher, i tried disconnecting one and then the other. all it did was made the vacuum rebound slower (on the gauge) thanks for the thought.
flash i am pretty sure that the booster/master are mating correctly, they were bought as a package so they were adjusted to fit before i got them. i think my next step is to grab that booster today and throw it in just to see if it helps.

flashgordon

Check the rod when you do it!........At this point you can't, take any thing for grantit!

Micaiahfied

ok so the new booster seems to have fixed it, power is awesome, almost went through the windshield the first time i hit it, (still used to stoping without the power) after that i tore the front brakes apart. turning the glazed rotors, replacing calipers, and pads. i figured since everything else was new (and i mean everything) i'd go ahead and do this as well. i'll be putting it all back together tomorrow and i'll let you guys know how it goes.

flashgordon

Grate job!!!!
I take it the rod was not holding the master sylinder from booster.
Did you notis any diff between rod lenghts of the new new booster compared to the old one?

Micaiahfied

not really, i think the new one was about 1/2 turn shorter, i just left it the way it was and all worked well. i think i still have some adjusting to do on the system, i can't believe it but i think the rears are still out of adjustment. just can't seem to get rid of the somewhat low pedal. i guess i will jack the back up again and give them each a few more clicks and see where it gets me.

edwinsmith

I've bought quite a few rebuilt parts over the last 35 years or so. Many times I've gotten defective parts. The most resent bad part was a master cylinder for my cheby.

Most of this stuff is rebuilt for the lowest cost possible to make the best profit. Most of the time it isn't tested. So yuh pays yo money an takes yer chances.

Edwin

wannadiesel

Mark, I'm glad you finally got it working right. Probably not half as glad as you, though.