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I think I have a mis-match here. The pushrod assy from the booster on the car is the top one in the pic. Anyone know if that booster is for later 78+ cars or?
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My preference would be to have the year appropriate combo on the car, but since the motor isn't, at this point I'd go for changing the master for whatever fits this booster. I don't really want to revisit that booster r&r...
Prob is, when bolting the master up the pushrod pushes the brake piston in about 1/4 inch or more. Brake pedal feels ok with the car off, with it running there's a rock hard pedal about 1/4 the way down and the car won't move. Shut the car off and brakes free up.
slofut, I've never seen the top item you're referring to before. The bottom rod is what I would expect to see, like the one in this image.
It does sound as if the booster is being activated by the "valve rod" before you even depress the brake pedal. Have you tried disconnecting the vacuum to the booster and seeing if the brakes react the same? If you disconnect the vacuum and start the engine and they function normally (as a non-PB system would), that would seem to confirm that the valve rod is either too long, or the booster is defective.
If they are still locking up, then the "hydraulic push rod" is too long.
FWIW , I put one of those boosters on my 69 , there is no adjustment on the pedal end (not enough thread to for the clevis to move ) . My master cylinder is a cast iron type , not sure what year ,but I had to adjust the front rod to get it right , the rod is right back in the plastic housing,
Thanks much for the replies everybody.
Gusto, I checked the old booster is indeed like what you describe. I'm pretty sure I ended up with a 77^ booster like Sharkbit and Bazza.
The reaction disc is a rubber disc about 1/4 in long, looks to be there to absorb shock and make things quieter. Think I'll bolt it up sans this disc and see what difference I get. I'm thinking it'll come close to alleviating the problem, then I can make adjustments from there, shorten the rod, make a spacer, etc... Worst case, I can get a 77 master and lose the bench bleed ports. If I fire it up with the vacuum disconnected it should work as it does with the engine off, which seems fairly normal. I'll try it.
Hey, another question... are the line ports the same size on the 77 and later masters? Can o worms again...
FWIW , I put one of those boosters on my 69 , there is no adjustment on the pedal end (not enough thread to for the clevis to move ) . My master cylinder is a cast iron type , not sure what year ,but I had to adjust the front rod to get it right , the rod is right back in the plastic housing,
I think GUSTO is right in his assessment.
Bazza, the rod goes back into the plastic housing all but about 1/4 inch. This would require trimming the plastic back about 1/2 inch which looks like what you may have done in your pic. Correct?
So there should be no preload of the master cyl piston when bolting it up correct? And the rod should obviously seat into the front and back connections at rest without pushing the master cyl piston in at all?
No my rod is right back in the plastic , I never cut the plastic part . I added on some rod as I had no brakes! I originally added too much and the car wouldnt roll with engine off , just kept filing it back down until I could feel some give on the pedal. Easy way to see how much needs to come off is to loosen the master slightly until car is movable , then just a bit more . Brakes worked fine until I pulled the car apart! Differant type master cylinder from yours is all.
That is a 77-82 reproduction booster made by MBM. However it is the same booster they use for the 68-76. Nobody has ever reproduced the original 68-76 booster, what MBM did is ship it with two different sized rods depending on what MC you use. If you have a 68-76 you use the long rod, if you have a 77-82 you use the short rod, make sure you have the correct mastercylinder because this is where the issue usually is.
That is a 77-82 reproduction booster made by MBM. However it is the same booster they use for the 68-76. Nobody has ever reproduced the original 68-76 booster, what MBM did is ship it with two different sized rods depending on what MC you use. If you have a 68-76 you use the long rod, if you have a 77-82 you use the short rod, make sure you have the correct mastercylinder because this is where the issue usually is.
Hi J,
Thanks much for the help, I'm almost sure I got the booster from you. Prob the master too, but it's been a couple years.
But this is interesting, when I got the master off the booster I noticed that it has what looks to be an adapter in the end of the piston maybe to adapt the deep hole to a flush end. Is that possible? and could I pull that out? Even at that, the plastic/metal pushrod assembly isn't going to work even with the deep hole. That's a dilemma in itself, the booster is not for a deep hole master and my master doesn't have a deep hole!
Not really sure what you are trying to explain. The booster is the same, they come with a long or short rod. The pastic rod holder goes into the booter and either the long or short rod goes into the end of the plastic plunger. If you have a 77-82 master cylinder you use the short rod, if you look at the end of the mastercylinder piston it just a has a shallow dimple in it. If you have a 68-82 you use the long rod. The master cylinder has a deeper hole to accommodate the rod.
Not really sure what you are trying to explain. The booster is the same, they come with a long or short rod. The pastic rod holder goes into the booter and either the long or short rod goes into the end of the plastic plunger. If you have a 77-82 master cylinder you use the short rod, if you look at the end of the mastercylinder piston it just a has a shallow dimple in it. If you have a 68-82 you use the long rod. The master cylinder has a deeper hole to accommodate the rod.
J,
My master has a shallow piston, but with a piece in the hole to make it that way, looks like it could come out to reveal a deep hole, maybe they're making them to work in both applications?
So, I do have the booster as shown above with the plastic piece and a short rod (only, no long rod came with it) and the master has the shallow recess in the piston. But I still have this problem. Hope to drive it this week without boost and ponder the problem in the meantime. BTW... The (short) rod I have is too short to cut off any further, it sticks out of the plastic piece only about 1/4 inch.
Oh, and if the problem was the pushrod to the pedal, wouldn't I have the hard pedal and brakes locking even with the car not running?
Thanks Bazza, of course I wont take any chances with the car. No brakes, no test drive.
I did learn today that the insert will come out of the master piston, and the long pushrod will fit into the booster, so it looks like it's a universal fit with both components.
Next step, I put it back together and will check the pushrod at the pedal for adjustment. I have a feeling thats where the solution lies.
Thanks Bazza, of course I wont take any chances with the car. No brakes, no test drive.
I did learn today that the insert will come out of the master piston, and the long pushrod will fit into the booster, so it looks like it's a universal fit with both components.
Next step, I put it back together and will check the pushrod at the pedal for adjustment. I have a feeling thats where the solution lies.
if you got the measurements for the booster rod protrusion and the master recess and they check out then what's left ?
hopefully the pedal ends up in the right spot when you hook it all back up because isn't that the trouble , there is no adjustment at the pedal .
Also worse case when its hooked up and if its still grabbing then the booster is no good ?
it wouldn't be the first time a new booster is no-good straight up.
I had one of the small space saver 6inch ones, no good out of the box , drove me nuts trying to work it out !! Even when they work they are still a poor performance unit.
Brake booster/master mismatch
