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  Mustangsandmore Forum Archive
  '69 to '73 -- The Musclecar Mustang
  How to test power brake booster?

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Author Topic:   How to test power brake booster?
Gary
Gearhead

Posts: 726
From: Sarasota, Fla. USA
Registered: Jun 99

posted 11-17-2001 12:16 PM     Click Here to See the Profile for Gary        Reply w/Quote
Can't figure out why my brakes are lousey, I have very little brakes and pedal is as hard as a rock. My engine doesn't make alot of vac. with the cam but have installed a vac. canister to asist. No leaks in the vac. lines.
How do you know when a booster is bad? anyway of testing it?
The brakes act like it would if the engine quits when it's still running..Hummm?
Thanks

Brakes have been blead and level is fine.
Both check valves are new too..

[This message has been edited by Gary (edited 11-17-2001).]

7DMACH1
Gearhead

Posts: 2915
From: PHILA. PA.
Registered: Feb 2001

posted 11-17-2001 01:09 PM     Click Here to See the Profile for 7DMACH1        Reply w/Quote
First I would make sure that you have vacuum at the booster. Pull the check valve and see if there is vacuum. Then check the check valve that it's holding, only should let vacuum flow one way. Then I would let the car run for a minute or two then shut it off. Pull the check valve, there should be a rush of vacuum, if not and the check valve is good, either the booster has a ruptured diaphram and won't hold vacuum or the engine is no making enough vacuum. Another thing you can try, take a pair of vise grips and crimp the vacuum hose going to the booster and see if the idle changes, because if the diaphram is ruptured in the booster this would cause a vacuum leak and the idle should change.If there is no difference in the idle then I think your cam is to big to run power brakes unless you use and auxilary pump like some people have done from a Caddy Deisel. RAY

------------------
RAY . FLY THE FLAG, SHOW THE WORLD WE ARE PROUD AMERICANS!70 MACH 1 351C NOW 408 STROKER 4SPD.
86 MERCURY CAPRI 5.0 RUNS 12.86 @107 1.65 60 ft.
2001 LINCOLN LS V8 1997 MUSTANG
SHOW THE WORLD, LIGHT A CANDLE.
http://www.geocities.com/slariviere/7DMach1.html

[This message has been edited by 7DMACH1 (edited 11-17-2001).]

SteveLaRiviere
Administrator

Posts: 48752
From: Saco, Maine
Registered: May 99

posted 11-17-2001 01:26 PM     Click Here to See the Profile for SteveLaRiviere        Reply w/Quote
You test to see if it will hold vacuum with a Mity-Vac. http://www.mityvac.com/

How many inches of vacuum do you have at the booster end of the hose?

What kind of shape is the check valve in?

------------------
'70 Mustang Mach 1 M code 351C 4V/FMX/3.25 open
'72 Mustang Sprint Coupe 351C 4V/FMX/4.30 Trac Lok
'94 F-150 XL 5.8L/E4OD/3.55 Limited Slip
'97 Probe GTS 2.5L DOHC
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I only want two things from life: I want patience and I WANT IT NOW!!!

Dave Gibson
Moderator

Posts: 10769
From: Norfolk, Virginia, USA M&M#166 MCA#47921
Registered: Aug 99

posted 11-17-2001 05:37 PM     Click Here to See the Profile for Dave Gibson        Reply w/Quote
Gary,
We have power brakes in the '65. We also have to use a vacum canister due to low vacum caused by the cam. Both Ray & Steve have given you several great things to check. If I recall right, the check vavle is notorious for breaking. Then the diaphram would be your next item to check.

Dave & Terri

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Poison Ivy (Dave's)
'65 Fastback Modified 289 4V, C-4, dual exhaust, styled steel wheels, rally pac, A/C
Blue Belle (Terri's)
'66 Coupe, Silver Blue Metallic, 351W 2-V, 4 speed, A/C, future sleeper.

7DMACH1
Gearhead

Posts: 2915
From: PHILA. PA.
Registered: Feb 2001

posted 11-17-2001 10:32 PM     Click Here to See the Profile for 7DMACH1        Reply w/Quote
Gary, if you discover the booster is bad make sure the back of the master cly. is dry. This is what sometimes causes the booster to go. If brake fluid gets on the diaphram it's done. RAY

------------------
RAY . FLY THE FLAG, SHOW THE WORLD WE ARE PROUD AMERICANS!70 MACH 1 351C NOW 408 STROKER 4SPD.
86 MERCURY CAPRI 5.0 RUNS 12.86 @107 1.65 60 ft.
2001 LINCOLN LS V8 1997 MUSTANG
SHOW THE WORLD, LIGHT A CANDLE.
http://www.geocities.com/slariviere/7DMach1.html

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