'67 Chevelle SS-396 Power Brakes
04-08-2013, 01:35 PM
#1
Team Owner
Thread Starter
'67 Chevelle SS-396 Power Brakes
Hate to impose but do any of you Chevelle guys (DZAuto, Nowhere Man) know if there was a rod between the power brake booster and master cylinder ?? The '67 Chassis Manual has one terse page on power brakes and not much info. I confirmed a leaky diaphragm and pulled the booster and lo and behold ... no rod. I don't see how the booster actuates the M/C piston. The pics are of the front of the booster and the back of the M/C (the little mirror view).
04-08-2013, 01:54 PM
#2
Race Director
Frank -
I'm no help on the MC rod, but am wondering if the leaking diaphragm may have/be contributing to your idle/off idle problems as discussed in another thread re the Carb? Does this unit have a check valve at the MC/vacuum hose connection? Have you checked to make sure it's functional? If not there or not functional you could be losing a lot of engine vacuum ( and having resultant symptoms) thru that leaky diaphragm.
I'm no help on the MC rod, but am wondering if the leaking diaphragm may have/be contributing to your idle/off idle problems as discussed in another thread re the Carb? Does this unit have a check valve at the MC/vacuum hose connection? Have you checked to make sure it's functional? If not there or not functional you could be losing a lot of engine vacuum ( and having resultant symptoms) thru that leaky diaphragm.
04-08-2013, 02:00 PM
#3
Team Owner
Thread Starter
Its the weirdest idle problem I've ever encountered and I'm scratching my head. If I step on the brake the idle drops 250RPM and goes pig rich (like 11.2 A/F ratio). This car has great idle vacuum (20" @ 750RPM).
Replaced the vacuum hose AND check valve - no help. Then, on someone's suggestion, I put vice grips on the vacuum line to the booster and the same thing happened (please tell me how that works ?)....which makes no sense to me.
Did the classic checks -- with foot on brake I start the car and the pedal goes from rock hard to correct and sinks about an inch (vacuum working). Turn the car off and the pedal is immediately at the top of its travel and rock hard. No vacuum reserve which should give you 2-3 good pumps with vacuum boost after engine shutdown.
Only thing I can figure now is the booster is shot - so.... new one on the way.
UPDATE: Called NPD vendor and they said no rod between M/C and booster. I guess the 'boost' is FM (F*ing Magic)...
Replaced the vacuum hose AND check valve - no help. Then, on someone's suggestion, I put vice grips on the vacuum line to the booster and the same thing happened (please tell me how that works ?)....which makes no sense to me.
Did the classic checks -- with foot on brake I start the car and the pedal goes from rock hard to correct and sinks about an inch (vacuum working). Turn the car off and the pedal is immediately at the top of its travel and rock hard. No vacuum reserve which should give you 2-3 good pumps with vacuum boost after engine shutdown.
Only thing I can figure now is the booster is shot - so.... new one on the way.
UPDATE: Called NPD vendor and they said no rod between M/C and booster. I guess the 'boost' is FM (F*ing Magic)...
Last edited by Frankie the Fink; 04-08-2013 at 02:03 PM.
04-08-2013, 02:31 PM
#4
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EVERY power brake system uses a pushrod from the front of the booster to the hole in the primary piston in the back of the master cylinder; some use a shorter rod, some use a longer rod (see photo below), but I've never heard of one that didn't use ANY rod.. 
04-08-2013, 02:55 PM
#5
Le Mans Master
M/C rod
Frankie: My 70 Chevelle has a short rod between the M/C and the booster. How else could it work. Suction? In fact, I have two different sizes, one is about a half an inch shorter than the other one. I forgot which one I used. Also as an update to my Chevelle, I could not get the vacuum up past 6". It turned out that the engine needed new lifters. When all lash was taken out of the valve train and a half turn more, the vacuum came up to 17". Of course the engine sounded like a sewing machine. The cam was fine. Near as I can figure, the engine builder must have soaked the lifters in oil before assembly and the oil must have dried out which resulted in the lifter not bleeding down. The engine did sit for a number of years before it was started. All is well now and I will post a picture when the car comes back from being cut and buffed. Jerry
In fact, I have two different sizes, one is about a half an inch shorter than the other one. I forgot which one I used. Also as an update to my Chevelle, I could not get the vacuum up past 6". It turned out that the engine needed new lifters. When all lash was taken out of the valve train and a half turn more, the vacuum came up to 17". Of course the engine sounded like a sewing machine. The cam was fine. Near as I can figure, the engine builder must have soaked the lifters in oil before assembly and the oil must have dried out which resulted in the lifter not bleeding down. The engine did sit for a number of years before it was started. All is well now and I will post a picture when the car comes back from being cut and buffed. Jerry
04-08-2013, 03:46 PM
#6
Team Owner
Thread Starter
Frankie: My 70 Chevelle has a short rod between the M/C and the booster. How else could it work. Suction? In fact, I have two different sizes, one is about a half an inch shorter than the other one. I forgot which one I used. Also as an update to my Chevelle, I could not get the vacuum up past 6". It turned out that the engine needed new lifters. When all lash was taken out of the valve train and a half turn more, the vacuum came up to 17". Of course the engine sounded like a sewing machine. The cam was fine. Near as I can figure, the engine builder must have soaked the lifters in oil before assembly and the oil must have dried out which resulted in the lifter not bleeding down. The engine did sit for a number of years before it was started. All is well now and I will post a picture when the car comes back from being cut and buffed. Jerry
In fact, I have two different sizes, one is about a half an inch shorter than the other one. I forgot which one I used. Also as an update to my Chevelle, I could not get the vacuum up past 6". It turned out that the engine needed new lifters. When all lash was taken out of the valve train and a half turn more, the vacuum came up to 17". Of course the engine sounded like a sewing machine. The cam was fine. Near as I can figure, the engine builder must have soaked the lifters in oil before assembly and the oil must have dried out which resulted in the lifter not bleeding down. The engine did sit for a number of years before it was started. All is well now and I will post a picture when the car comes back from being cut and buffed. JerryI've been driving this car for a year and there is NO rod between the M/C and power booster. I took those two pieces apart like delivering a baby and there was nothing there and nothing fell out. I'm freakin' mystified. I may take the old power booster apart to see if it fell inside somehow
04-08-2013, 03:58 PM
#7
Team Owner
Thread Starter
Thank JohnZ & Tampa Jerry - NPD changed their tune when I called back and "fired 'em up" about this. The booster should come tomorrow and SUPPOSEDLY has both push rods with it as a kit (I hope so). I'm tellin' you guys tho' there was NO push rod in what I took apart this morning - scary as hell...
Been driving the car (aggressively) for over a year now!
P.S. I'm convinced there must have been a rod i there I dropped or whatever. It's inconceivable the brakes would have worked otherwise - put the car up on ramps and felt around, used an inspection mirror and magnet and the rod simply vanished somehow!
Been driving the car (aggressively) for over a year now!
P.S. I'm convinced there must have been a rod i there I dropped or whatever. It's inconceivable the brakes would have worked otherwise - put the car up on ramps and felt around, used an inspection mirror and magnet and the rod simply vanished somehow!
Last edited by Frankie the Fink; 04-08-2013 at 05:31 PM.
04-08-2013, 05:59 PM
#8
Melting Slicks
Prediction for Frankie
next week, Frankie's #1 spouse will hand Frankie a push rod and ask him what it is after she found it banging around in the clothes washer....
04-08-2013, 06:40 PM
#9
Drifting
"If I step on the brake the idle drops 250RPM and goes pig rich (like 11.2 A/F ratio). This car has great idle vacuum (20" @ 750RPM)."
That's your clue Frankie! Without stepping on the brake, there's an air leak from the booster going to the engine and letting it rev up. Step on the pedal and you somehow stop the leak of extra air to the engine and it goes rich and has a lower idle. It could be a few things, bit maybe the rubber rear seal is a bit stiff and when the (lost) pushrod pushes on it, it somehow makes a good seal again? You may have bought a new booster when all you needed is a new rear seal?
(I hope you find your pushrod)
Verne
That's your clue Frankie! Without stepping on the brake, there's an air leak from the booster going to the engine and letting it rev up. Step on the pedal and you somehow stop the leak of extra air to the engine and it goes rich and has a lower idle. It could be a few things, bit maybe the rubber rear seal is a bit stiff and when the (lost) pushrod pushes on it, it somehow makes a good seal again? You may have bought a new booster when all you needed is a new rear seal?
(I hope you find your pushrod)
Verne
04-11-2013, 04:58 PM
#10
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St. Jude Donor '08
Team chevelle
Team Chevelle forum will tell you everything you will ever need to know ...Best forum I have found for Chevellesand could not have restored my 68 SS396 with out there help ..
04-12-2013, 10:09 AM
#12
Team Owner
Thread Starter
Sorry - but I don't share your high opinion of the Chevelle forum (and I am a member). I only come here when I couldn't get an answer over there or the answers were obvious BS.
Anyway - changed out brake booster today (PITA) and that was indeed the problem. Idle doesn't budge with brakes applied now and, even better, the little 'shudder' the car always had at an idle is gone !! Drove the car back and forth and jumped up and down on the bumpers a bunch hoping to shake the old push rod loose from the undercarraige. Hasn't shown up anywhere, never saw it on disassembly so I don't know WTF is up with that. There IS a push rod in there now and the brakes are freakin' awesome.
Anyway - changed out brake booster today (PITA) and that was indeed the problem. Idle doesn't budge with brakes applied now and, even better, the little 'shudder' the car always had at an idle is gone !! Drove the car back and forth and jumped up and down on the bumpers a bunch hoping to shake the old push rod loose from the undercarraige. Hasn't shown up anywhere, never saw it on disassembly so I don't know WTF is up with that. There IS a push rod in there now and the brakes are freakin' awesome.
04-12-2013, 05:52 PM
#14
Race Director
Anyway - changed out brake booster today (PITA) and that was indeed the problem. Idle doesn't budge with brakes applied now and, even better, the little 'shudder' the car always had at an idle is gone !! Drove the car back and forth and jumped up and down on the bumpers a bunch hoping to shake the old push rod loose from the undercarraige. Hasn't shown up anywhere, never saw it on disassembly so I don't know WTF is up with that. There IS a push rod in there now and the brakes are freakin' awesome.
Frank -
Glad to hear that you found a solution to both problems!
Glenn