checking mastercylinder push rod length
Le Mans Master
Joined: Aug 2001
Posts: 7,804
Likes: 4
From: Standing still with all my might Texas
Cruise-In X Veteran
St. Jude Donor '09-'11
Here is the Booster that I took off of Liz's 68.
I don't remember the thread size of the studs, but the nut size is 17mm. This booster did not leak at all. The electric vacuum pump was delivering at least 20" of vacuum to it, but I almost had to stand on the brake pedal to lock the brakes up.
I replaced it with a correct, rebuilt booster from Orielly and the brakes are now perfect.
I don't remember the thread size of the studs, but the nut size is 17mm. This booster did not leak at all. The electric vacuum pump was delivering at least 20" of vacuum to it, but I almost had to stand on the brake pedal to lock the brakes up.
I replaced it with a correct, rebuilt booster from Orielly and the brakes are now perfect.
Thread Starter
Burning Brakes
Joined: Aug 2005
Posts: 801
Likes: 1
The one I have came from a Corvette parts supplier.It dosent look like that where the rod screws on to the clevis.It has a round acordian looking boot around the clevis rod.the othere side where the mastercylinder bolts up If I remember it right there wasn't anything sticking out of that hole in the way of a rod.when I installed the rid that went into the mastercylinder it went in the booster about an inch but don't quote me its been almost a year since I converted it over.The body of the booster looks about the same as mine but I can't tell about the diameter.I did fire it up this afternoon and it had 17inches of vaccum at idle and 20 just above idle.
Last edited by rugerm44; Mar 9, 2011 at 10:09 PM.
Thread Starter
Burning Brakes
Joined: Aug 2005
Posts: 801
Likes: 1
Thats the way mine looks on the inside of the car where the shaft comes out and attaches to the clevis.just don't remember the rod sticking out from the other side.could be because it was new and had been depressed by a brake pedal at that point.In going to figure this out even if it kills me.remember anything strange about the size of the nuts on the mastercylinder mounting stud. I wonder if yours had metric studs,also.
Last edited by rugerm44; Mar 9, 2011 at 10:27 PM.
Le Mans Master
Joined: Aug 2001
Posts: 7,804
Likes: 4
From: Standing still with all my might Texas
Cruise-In X Veteran
St. Jude Donor '09-'11
Thats the way mine looks on the inside of the car where the shaft comes out and attaches to the clevis.just don't remember the rod sticking out from the other side.could be because it was new and had been depressed by a brake pedal at that point.In going to figure this out even if it kills me.remember anything strange about the size of the nuts on the mastercylinder mounting stud. I wonder if yours had metric studs,also.
The booster that I replaced it with had SAE threads all around. The original booster on my 73 had SAE threads, and the new replacement booster I bought for my 73 had SAE threads.
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Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
Here is the Booster that I took off of Liz's 68.
I don't remember the thread size of the studs, but the nut size is 17mm. This booster did not leak at all. The electric vacuum pump was delivering at least 20" of vacuum to it, but I almost had to stand on the brake pedal to lock the brakes up.
I replaced it with a correct, rebuilt booster from Orielly and the brakes are now perfect.
I don't remember the thread size of the studs, but the nut size is 17mm. This booster did not leak at all. The electric vacuum pump was delivering at least 20" of vacuum to it, but I almost had to stand on the brake pedal to lock the brakes up.
I replaced it with a correct, rebuilt booster from Orielly and the brakes are now perfect.
Your pics show the later 82 style booster (metric) with the earlier style long pushrod.
Nothing wrong with that combo. The master doesn't care and the booster doesn't care as long as the correct length rod is used. You can mix any of the masters/boosters/rods from 68 to 82.
The booster you took from Liz's 68 was simply defective.
They fail for other reasons internally than just vacuum leaks.
The pics I posted earlier were from an older long rod booster with a new short rod piston master using the lower short rod.
The reason I mismatched them was to use a later 1-1/4" master cylinder.
Le Mans Master
Joined: Aug 2001
Posts: 7,804
Likes: 4
From: Standing still with all my might Texas
Cruise-In X Veteran
St. Jude Donor '09-'11
Like I said earlier the. GM changed to metric boosters in 82.
Your pics show the later 82 style booster (metric) with the earlier style long pushrod.
Nothing wrong with that combo. The master doesn't care and the booster doesn't care as long as the correct length rod is used. You can mix any of the masters/boosters/rods from 68 to 82.
The booster you took from Liz's 68 was simply defective.
They fail for other reasons internally than just vacuum leaks.
The pics I posted earlier were from an older long rod booster with a new short rod piston master using the lower short rod.
The reason I mismatched them was to use a later 1-1/4" master cylinder.
Your pics show the later 82 style booster (metric) with the earlier style long pushrod.
Nothing wrong with that combo. The master doesn't care and the booster doesn't care as long as the correct length rod is used. You can mix any of the masters/boosters/rods from 68 to 82.
The booster you took from Liz's 68 was simply defective.
They fail for other reasons internally than just vacuum leaks.
The pics I posted earlier were from an older long rod booster with a new short rod piston master using the lower short rod.
The reason I mismatched them was to use a later 1-1/4" master cylinder.
Admittedly I have never really studied the internal workings of a brake booster, I did not know that they could be defective and still hold a vacuum. I have always been told that as long as the booster will hold a vacuum it is OK.
Learn something new every day.
Thread Starter
Burning Brakes
Joined: Aug 2005
Posts: 801
Likes: 1
Sounds like Im the proud owner of a 77-82 power brake booster.the kit came a short rod and a longer rod to go behind the mastercylinder.I put the longer rod in it.knowing all this what are my options.Im not wild about mixed match parts.kevin you said yours had the metric booster on it before you replaced it with the earlier one.what kind of mastercylinder did it have on it.was it the one with the shorter piston made go the 77 up.lm willing to go back in and change the booster out yet again if I have too.just want to make damn sure I get eveything right this time.this is depressing.
Thread Starter
Burning Brakes
Joined: Aug 2005
Posts: 801
Likes: 1
Just for the heck of it. I typed in the part number of the kit they sent me on their website .It states 76-86 power brake booster.I don't really understand why people can't just sell you the right parts.
Safety Car
Joined: Apr 2008
Posts: 3,908
Likes: 10
From: Charleston SC
Does anybody know if the new MCs from VB&P need adjusting, if there is such a thing for them, or are they good out of the box?
I have manual brakes on my 327 '68 and I simply swapped out the MCs. I didn't adjust anything, or change anything with it. I also changed out the calipers/pads/rotors, and short lines at the calipers. Just want to make sure that I should have adjusted something when I swapped MCs..
I have manual brakes on my 327 '68 and I simply swapped out the MCs. I didn't adjust anything, or change anything with it. I also changed out the calipers/pads/rotors, and short lines at the calipers. Just want to make sure that I should have adjusted something when I swapped MCs..
Thread Starter
Burning Brakes
Joined: Aug 2005
Posts: 801
Likes: 1
If its got a good firm pedal without a lot of travel.Id say you are in better shape than me.I just got off the phone with Corvette central.the lady said she would have pull the correct booster and check the size of the nuts on it and get back to me.nice lady but dosent seem to know much about the cars othere than part numbers.oh well looks like another 15 hours worth of fun.when I get over this damn flu I will swap it out and get it over with.
Thread Starter
Burning Brakes
Joined: Aug 2005
Posts: 801
Likes: 1
I just an email back from Corvette central.the guy says that all of the brake boosters they sell in their conversion kits have metric threads and come with a long and a short rod depending on which mastercylinder you use.thats funny I called paragon and was told theres has standard threads on their 68-76 boosters.question is 17 to20 lbs of vacuum enough to run a booster?I don't want change this thing for fun but if it will get me the correct pedal feel I will.Its just funny I had to really push down hard to get a disturbance in the front brake reservoir with lid off the mastercylinder.this is without the Eng running.
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
I just an email back from Corvette central.the guy says that all of the brake boosters they sell in their conversion kits have metric threads and come with a long and a short rod depending on which mastercylinder you use.thats funny I called paragon and was told theres has standard threads on their 68-76 boosters.question is 17 to20 lbs of vacuum enough to run a booster?I don't want change this thing for fun but if it will get me the correct pedal feel I will.Its just funny I had to really push down hard to get a disturbance in the front brake reservoir with lid off the mastercylinder.this is without the Eng running.
Unless you are having it judged by Bloomington or NCRS or others or if you don't like the looks or the thought of it…. change it. Please understand it doesn't make any difference whatsoever as far as brake operation goes which booster/master you use and 90% of the people looking at it installed can’t tell.
I suggest going thru the proper test to determine what the actual problem(s) are.
A hard pedal a little more than half way to the floor with engine running is normal for this old GM design.
Old long rod style SAE threaded booster NAPA NBB5491200
Racer
Joined: Dec 2008
Posts: 297
Likes: 19
From: arlington TX
I went through something similiar with replacing my master cylinder on my 74. Check out my thread in this forum "1974 master cylinder replacement - WTF". It may help. Turns out that my PB and MC were from a 77 and up Corvette. Also, yes the nuts are 17mm. Yes, 17 INCHES of vacuum is plenty.
Good luck.
walt
Good luck.
walt
Thread Starter
Burning Brakes
Joined: Aug 2005
Posts: 801
Likes: 1
I'm just after the correct pedal travel.the way it feels now if it were to leak any fluid you wouldn't have a hell of a lot of extra travel to stop it.If it worked great with what's on there I would be tickled pink.
Thread Starter
Burning Brakes
Joined: Aug 2005
Posts: 801
Likes: 1
I went and put the 3rd new mastercylinder on it.bench bleed for about 20minutes.a good 15 strokes after air stop coming thru it.the booster looks like the one in your picture.It dosent have adjustable rod just a metal disc with a phenolic guide for the rod.I ran out of time to power bleed it.I did measure the pedal.it sits 3 1/2 off the floor.I don't know if that's correct for a power brake car or not.I will know more about it after I bleed the system.
Safety Car
Joined: Oct 2005
Posts: 3,570
Likes: 1
From: Algonac Michigan
"I don't want change this thing for fun but if it will get me the correct pedal feel I will.Its just funny I had to really push down hard to get a disturbance in the front brake reservoir with lid off the mastercylinder.this is without the Eng running."
If you push the pedal without the engine running there is NO power assist and it will feel hard!!!
If you push the pedal without the engine running there is NO power assist and it will feel hard!!!
