Engine/Transmission (1994 - 1998) Diagnosing Loss Of Power Assisted Brakes

[DMC]

The other day I noticed some loss of power assist on my brake system, And got to thinking its time to use that brake fluid I bought about six months ago.



Changed the fluid in the reservoir, bled the brakes in the proper way. And still have the hard to push pedal after more than four or five applications of the brakes. When the pedal gets hard to push the low vacuum sensor, trips the Brake And ABS light on the dash. Then if I put the vehicle in park or increase engine rpms the lights go out and the cycle will repeat.



I hooked up a vacuum gauge and checked the system at the point where the AC vacuum lines pass through the fire wall and had 10 in/hg of vac at idle.



I then blocked off the AC controls that pass through the firewall, from the rest of the system, and have 25 in/hg of vacuum at idle, and the system will hold that pressure for quite some time, after shut down.



I pulled some of the dash away to look for problems with the vac lines under the dash. I found that the actuater for the floor vent is leaking, ex: pump the vacuum gauge up to 20 in/hg and it drops down to 5 in/hg, then slowly bleeds off. For comparison purposes, the mid level vent will hold 20 in/hg vacuum when tested.



Also I noticed that when I pull the check valve on the booster, it had quite a bit of vacuum remaining on it.



I am wondering if I can block off the vacuum lines that run into the firewall for the ac controls, and test the brakes by running it that way, to see if the power brakes will work properly.



If the floor vent actuator is bad, does anyone have a part number?



Any suggestions?



Thanks



Don

 

[Extreme1]

The vacuum brakes do loose their power assist at idle after a few applications, that's normal. It is also why they went to the hydroboost system, vacuum then is used for AC and cruise only.



As for the vacuum brake, are you refering to the in the center of the dash on the bottom of the HVAC unit?

 

[DMC]

The actuater that controls the floor vent, when in the floor mode, is loosing vacuum.



My truck always had good brakes until last week.



I think you may have missunderstood my post I thought I explained the situation with complete detail.



Thanks



Don

 

[DMC]

Back to the top

 

[DMC]

Does anyone know the part number for the vent actuator, I discovered that this may be the culprit. When holding the brake pedal down (motor running) about the same time that I feel the pedal trying to push up, the actuater releases, sending the flow of air through the defrost vents. This actuator rapidly looses vacuum when tested, and only holds 10 in/hg when the motor is running. I think I found the problem with the other actuater, I think it was hooked up backwards. Red vac line should be on the front, and the black line on the back.



Any AC Techs out there.



Thanks



Don

 

[DMC]

Well it doesn't look like I'm going to get much help on this issue from the brake gurus at TDR.



I installed the new vent defrost actuator, and still have problems, with the hard pedal. Brake booster seems to be bleeding down when it is hooked up to the vac gauge. So I bought a brake booster today, will install it tomorrow. 100 bucks at Napa, will get $30. 00 back when I return the bad one.



I guess I'll just keep throwing parts at this problem until I fix it.



At least I will post the outcome, So the next guy that has these problems might have the benefit of someone putting the pieces up on the boards.



Thanks for all the interest guys.



Have a nice day



Don

 

[DMC]

New (Reman) Brake booster is in, and the hard pedal issue is gone, "Thank God". I do however agree with those who say that a Vacuum accumulater tank would be helpful. Since replacing the Brake booster, I can hold the brake pedal down indefinately with no loss of vacuum. I did notice one thing, when you pump the brakes say 10 times when sitting still the system will bleed down causing the vent /defrost vent to switch to defrost as it did before. But if you hold steady pressure on the brake pedal the vacuum will return to the proper level, opening the vents. It would not do this before I changed the brake booster.



Brakes are once again reliable.







For anyone wondering about this job. Its not as bad as they describe it to be in the manual. You do not have to disconnect the the brake lines from the Master cylinder. Just unplug the electrical sensors, and ground wire, tuck them out of the way, diconnect the vacuum line from the booster, remove the two 15mm nuts that hold the master cylinder to the booster, put the master cylinder down in the space by the intake tube. Remove the kick panel inside of the truck, remove the brake light switch, take the clip off the brake pedal, remove the 4 15mm nuts that hold the brake booster in place, remove the booster. Reverse the procedure for installation, Takes about an hour and a half if you really take your time. If you see any brake fluid in the cavity between the master cylinder and the brake booster, then you need to replace or rebuild the master cylinder.



Good Luck



Don