Brakes slightly sticking with B body master and booster

Squag27

Not-quite-so-new-guy
Jun 23, 2021
33
17
8
West Palm Beach FL
Hey guys, haven't posted in a while but I could use some input from others who've done the swap. Nowadays I run Blazer discs front and rear as well as a B-body master. Recently I tried swapping over to a B body booster as well but in doing so it seems like the brakes will stick ever so slightly. I thought maybe the booster was an issue so I returned it and tried from another place with no difference. I have a modified brake pedal setup for my manual swap so checked and confirmed that there's nothing hanging up on the pedal side too. Also I replaced the proportioning valve with an aftermarket disc/disc unit, and separately even tried swapping the master for a spare RHD S10 blazer master I have from another G body project. Neither of those made a difference. It almost seems like the pushrod in between the booster and master is slightly too long. Problem is that both the booster and master are from a 1990 Caprice so I can't see how that would make a difference. Does anyone have any ideas on where to go from here? I haven't seen of anyone else online with this issue and asides from swapping back to my worn out original booster, I'm out of ideas as to where to go from here.
 

Squag27

Not-quite-so-new-guy
Jun 23, 2021
33
17
8
West Palm Beach FL
Good point. I do need to get it jacked up and see which is exactly sticking but to answer your question, yes it is bled correctly. Swapping back in my old original booster makes the issue go away so it's more than likely not an issue downstream of the master.
 

JimmyD

Apprentice
Feb 16, 2021
53
32
18
Umatilla, Florida
You can use some flat washers 'temporarily' between your master and the new booster to verify if the booster rod is to long.
I have seen adjustable rods on some boosters, not sure about yours.
 

Ribbedroof

Comic Book Super Hero
Supporting Member
Jan 4, 2009
4,861
6,888
113
Wellston, OK
Sounds to me like the booster pushrod is too long and is pre-loading the master cylinder. As mentioned above some of them are adjustable and need to be adjusted prior to installation of the master cylinder
 
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ck80

Moderator
Moderator
Supporting Member
Feb 18, 2014
5,742
9,111
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Sounds to me like the booster pushrod is too long and is pre-loading the master cylinder. As mentioned above some of them are adjustable and need to be adjusted prior to installation of the master cylinder
Another possibility here is its a combination of 2 things..

You said you have custom pedal setup, and, then you said your old booster was worn out. It could be there was enough stretch / wear / loss of rigidity of the diaphragm in the old booster that the pushrod was slightly pushed towards the master without corresponding preload into the master that it offset the geometry of the custom pedal setup being just a smidgen closer to the firewall than a stock brake pedal would.

Picture it like this, using numbers:

Stock:
Master cyl----- 5units output///tight diaphragm 5 units pressure/// rod to pedal 5 units.

Your old setup :
Master cyl----- 5units output///worn diaphragm 4 units pressure/// rod to custom pedal 6 units pressure

With new booster:
Master cyl----- 6units output///good diaphragm 5 units pressure/// rod to custom pedal 6 units pressure

Instead of the worn diaphragm taking up slack and not passing through to preload, now there's no give in the middle before pressure gets generated.

Just a thought???
 

Squag27

Not-quite-so-new-guy
Jun 23, 2021
33
17
8
West Palm Beach FL
Well as an update, the pushrod in the booster is for sure too long for the master I actually even removed the booster from the car and fit it up to the master to 100% rule out the pedal assembly. Super weird considering they're both supposed to be for a '90 Caprice. Well fortunately it's nothing that chucking the pushrod up in a lathe can't fix. It's close though, probably only need to take 0.060" off or so.
 
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uneek1976

G-Body Guru
Jul 24, 2009
653
129
43
Sugar Grove IL/ Chicago
I have recently swapped my booster for the caprice one, I already had the caprice mc installed and it seems like I have alot of travel in the pedal. But I had no issues with the rod from the new booster.
 

Squag27

Not-quite-so-new-guy
Jun 23, 2021
33
17
8
West Palm Beach FL
I have recently swapped my booster for the caprice one, I already had the caprice mc installed and it seems like I have alot of travel in the pedal. But I had no issues with the rod from the new booster.
There's a Caprice in the local yard I'm going to go check out this weekend for reference against my parts. Machining 0.060" off the pushrod did the trick though, the brakes feel perfect now and have just a smidge of travel before engaging. I forgot to measure ahead of time but after machining I believe the rod ended up being pretty close to 2.440" long.
 

oldsmobile joe

Royal Smart Person
Nov 12, 2015
2,067
3,053
113
mpls
have you compared the booster rod from your original booster to the rod from the caprice booster?
 

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