Incorrect Booster?
Thread Starter
Melting Slicks
Joined: Nov 2001
Posts: 2,793
Likes: 58
From: God's Country The Old Dominion
Incorrect Booster?
I picked up a master and booster from NAPA to convert my '68 to power brakes. The new booster has a rod sticking out of it that would go into the master cylinder. I have a core from a '79 that does not have the rod. Do I have the wrong booster? The AIM does not have a clear diagram but the one it does have does not show the rod. 
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
The rod actuates the master cylinder. Since you bought it as a 'matched set' (that IS the case, isn't it?), the rod should be the correct one for that master cylinder. But, the depth of the center hole in the M/C piston and the rod length must be matched if those two components are going to work properly.
When the master cylinder is mounted to the booster and gasket, the end of that rod should be within .010" of being bottomed-out inside that piston. It can be contacting at the bottom, but not actuating the piston. You can do a 'dry' measurement of how much the rod is protruding from the gasketed surface of the booster AND the depth of the piston actuating 'hole' from the mating surface at the base of the M/C. If those readings are not very close to each other, those components will not work with each other.
P.S. You can just pull the rod out of the booster, if you want. It is just held in there by the resistance of 1 or 2 O-ring seals.
When the master cylinder is mounted to the booster and gasket, the end of that rod should be within .010" of being bottomed-out inside that piston. It can be contacting at the bottom, but not actuating the piston. You can do a 'dry' measurement of how much the rod is protruding from the gasketed surface of the booster AND the depth of the piston actuating 'hole' from the mating surface at the base of the M/C. If those readings are not very close to each other, those components will not work with each other.
P.S. You can just pull the rod out of the booster, if you want. It is just held in there by the resistance of 1 or 2 O-ring seals.
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,754
From: Charlotte NC
There are 2 Boosters that GM used . On your 1968...your booster should have an adjustable center shaft in the booster...AND...this is important...when the master cylinder is joined to the booster...the metal surface on the booster is FLAT...so you can actually see that the master cylinder is seated against it when tightened. The outer half of the booster is stamped completely different than the later design...which makes it easier to access your oil dipstick without having to bend it.
On the later year Corvettes where the oil dipstick went to the passenger side...the booster design changed and when the master cylinder is attached to the booster...the area where the master cylinder goes is actually recessed...and there is no way to adjust the booster by turning a bolt.
The later design will work as long as the master cylinder is correct for it or the booster came with the optional center pin that can be changed out....but if your oil dipstick is on the drivers side (left) you may have to tweak it to make it easy to pull out.
DUB
On the later year Corvettes where the oil dipstick went to the passenger side...the booster design changed and when the master cylinder is attached to the booster...the area where the master cylinder goes is actually recessed...and there is no way to adjust the booster by turning a bolt.
The later design will work as long as the master cylinder is correct for it or the booster came with the optional center pin that can be changed out....but if your oil dipstick is on the drivers side (left) you may have to tweak it to make it easy to pull out.
DUB
Thread Starter
Melting Slicks
Joined: Nov 2001
Posts: 2,793
Likes: 58
From: God's Country The Old Dominion
There are 2 Boosters that GM used . On your 1968...your booster should have an adjustable center shaft in the booster...AND...this is important...when the master cylinder is joined to the booster...the metal surface on the booster is FLAT...so you can actually see that the master cylinder is seated against it when tightened. The outer half of the booster is stamped completely different than the later design...which makes it easier to access your oil dipstick without having to bend it.
On the later year Corvettes where the oil dipstick went to the passenger side...the booster design changed and when the master cylinder is attached to the booster...the area where the master cylinder goes is actually recessed...and there is no way to adjust the booster by turning a bolt.
The later design will work as long as the master cylinder is correct for it or the booster came with the optional center pin that can be changed out....but if your oil dipstick is on the drivers side (left) you may have to tweak it to make it easy to pull out.
DUB
On the later year Corvettes where the oil dipstick went to the passenger side...the booster design changed and when the master cylinder is attached to the booster...the area where the master cylinder goes is actually recessed...and there is no way to adjust the booster by turning a bolt.
The later design will work as long as the master cylinder is correct for it or the booster came with the optional center pin that can be changed out....but if your oil dipstick is on the drivers side (left) you may have to tweak it to make it easy to pull out.
DUB
I did not get one specific to a '68 as my car has manual brakes now. I assume the replacement one from NAPA is for many years. I will have to test fit it to see if it interferes with the dip stick. Thanks for the help.
Thread Starter
Melting Slicks
Joined: Nov 2001
Posts: 2,793
Likes: 58
From: God's Country The Old Dominion
The rod actuates the master cylinder. Since you bought it as a 'matched set' (that IS the case, isn't it?), the rod should be the correct one for that master cylinder. But, the depth of the center hole in the M/C piston and the rod length must be matched if those two components are going to work properly.
When the master cylinder is mounted to the booster and gasket, the end of that rod should be within .010" of being bottomed-out inside that piston. It can be contacting at the bottom, but not actuating the piston. You can do a 'dry' measurement of how much the rod is protruding from the gasketed surface of the booster AND the depth of the piston actuating 'hole' from the mating surface at the base of the M/C. If those readings are not very close to each other, those components will not work with each other.
P.S. You can just pull the rod out of the booster, if you want. It is just held in there by the resistance of 1 or 2 O-ring seals.
When the master cylinder is mounted to the booster and gasket, the end of that rod should be within .010" of being bottomed-out inside that piston. It can be contacting at the bottom, but not actuating the piston. You can do a 'dry' measurement of how much the rod is protruding from the gasketed surface of the booster AND the depth of the piston actuating 'hole' from the mating surface at the base of the M/C. If those readings are not very close to each other, those components will not work with each other.
P.S. You can just pull the rod out of the booster, if you want. It is just held in there by the resistance of 1 or 2 O-ring seals.
