Hard brake pedal / intermittent brake light after booster & MC replacement

Kizer

Supporting Member
Supporting Member
Joined
Mar 22, 2007
Posts
722
Reaction score
291
Location
Denton, Texas
Mornin'. A couple of days ago I replaced the brake booster and MC (purchased as a unit from NAPA) and I haven't been able to cure a hard brake pedal. My vacuum pump makes & holds 22" (per Mightyvac). In addition I get an intermittent flash of the BRAKE light while driving.
It feels like the booster isn't working like it should but I don't know how to test it.

Also, it would be great if one of y'all would help me understand what causes the BRAKE light to come on. I see an electrical connection at the "proportioning" valve and one on what appears to be a vacuum switch.

Thank you in advance. I'm sure that once y'all chime in I'll hear my head pop out.
By the way, this scenario is occurring on the '85 4x4 in my signature, if it matters.
 

DaveBen

Supporting Member
Supporting Member
Joined
Sep 29, 2010
Posts
1,916
Reaction score
633
Location
Ukiah, Ca
Hard brake pedal makes me think you have no vacuum at the booster. Check the vacuum line going the the booster from the vacuum pump.
 

DaveBen

Supporting Member
Supporting Member
Joined
Sep 29, 2010
Posts
1,916
Reaction score
633
Location
Ukiah, Ca
It could also be a bad booster, even if new or rebuilt.
 

Kizer

Supporting Member
Supporting Member
Joined
Mar 22, 2007
Posts
722
Reaction score
291
Location
Denton, Texas
Hard brake pedal makes me think you have no vacuum at the booster. Check the vacuum line going the the booster from the vacuum pump.
That's what I thought too. I tested the pump at the booster end of the hose... holds 22"
 

DaveBen

Supporting Member
Supporting Member
Joined
Sep 29, 2010
Posts
1,916
Reaction score
633
Location
Ukiah, Ca
If you have full vacuum at the booster and the pedal is still hard, it might be bad. I don't know how you can test this. You have a hose with vacuum going in, so it must be going out if the hose is clear and not cracked. Anybody else have an idea?
 

cpdenton

Truck needs paint.
Supporting Member
Joined
Aug 20, 2012
Posts
1,810
Reaction score
75
Location
Conway AR
Can you hook up your vacuum gauge and tee it in with the booster still hooked up? Booster might be a leaker.

Low vacuum(from a vacuum switch somewhere on an inner fender)and low fluid(per a switch in the MC) are what make the brake light come on.
 

Kizer

Supporting Member
Supporting Member
Joined
Mar 22, 2007
Posts
722
Reaction score
291
Location
Denton, Texas
Thanks y'all. I'll try the T in vacuum line idea.
I only see two electrical connections.. The vacuum switch on the fender and the "proportioning" valve.
Would air in that valve give me hard pedal and brake light?
 

OLDBULL8

Good Morning Ya'll.
Supporting Member
Joined
Feb 12, 2009
Posts
9,923
Reaction score
338
Location
Delphos , Ohio
Quote (It feels like the booster isn't working like it should but I don't know how to test it.)

with the engine off and running before to produce a vacuum, just take the check valve out of the booster, you should hear air rush into the booster if it's a good one.
IOf you still have the tomato can on the drivers side fender, it may have rusted out and the Vac pump can't keep up.
 

Kizer

Supporting Member
Supporting Member
Joined
Mar 22, 2007
Posts
722
Reaction score
291
Location
Denton, Texas
The T in the vacuum line proved that the "new" booster is a leaker.
I wasn't able to try pulling the check valve because there was a tag on the booster stating that the warranty would be void if the check valve was removed. I'm not clear on how they would ever know that I removed it but it wasn't worth it for me to risk the $.
Thank y'all again for the help.
 

Forum statistics

Threads
91,217
Posts
1,128,519
Members
24,045
Latest member
Ramtough01

Members online

No members online now.
Top