About a month ago, I started noticing something unusual with my power brakes. After initial startup and when first driving off and coming to a stop, the brake pedal travel is not as smooth as it used to be...it's kind of herky-jerky. Occasionally, when releasing the brake pedal to accelerate from a stop light, the brake pedal doesn't seem to release and the brakes feel like they're still applied. After about 10-15 miles of driving (mostly suburban stop and go) the brakes start to feel normal. During the remainder of my 40 mile drive, they feel perfectly fine. The weather here in the DFW area has been absolutely perfect driving weather over the last month, and I have put just over 1000 miles on the car, enjoying the ride while trying to figure out what could be causing this issue.

During the past month, I changed the oil and filter, as well as, doing a tire rotation. This gave me the opportunity to closely examine all four calipers and brake lines. Everything is bone dry...no seepage anywhere. The brake pads are approximately 1/4" thick. Both bowls of the dual master cylinder are filled to spec (1/4" below the back top edge). I bleed and flush the brake system each spring. Engine delivers a constant 15" Hg at idle.

Obviously, I don't want to start pulling things apart without a known solution. I'm looking for input from those who have experienced similar issues and what steps you took to diagnose the problem. My first thought was, possibly, the brake booster was failing or the vacuum check valve...but, I have no way of testing/diagnosing whether this could be the problem area.

Looking forward to helpful input. Thanks in advance

 

Leif, did you pull the vacuum line and check for brake fluid?

 

Leif
Have you tried depleting the all vacuum from the system by depressing the brake pedal several times with engine off? Then, apply brake pedal pressure & start the engine. Pedal should 'fall away' as vacuum boost returns. If not, suspect a problem somewhere with/within the booster.
Probably the only non-invasive component that could be swapped readily would be the check valve. Examine the vacuum line for fluid while in there. Any white exhaust smoke present?
Jeff

 

Mark,
Removed vacuum line from check valve...dry as the desert. Air passes through in only one direction.

 

Jeff,
Pumped the brake pedal, with car off, five to six times...pedal stays firm within an inch of the top of travel. Start the car with brake pedal applied, brake pedal doesn't move from initial position. No fluid in vacuum line. No white exhaust smoke...or smoke of any kind.

 

You said It feels like the brakes are not releasing, that sounds like calipers hanging up. Check to see if your rims are warm/hot after driving the car. Check things like a vacuum leak between the intake and the booster and the check valve. Warm up motor, turn motor off and pull check valve/hose off booster. Did the booster still have vacuum in it when the hose is pulled off?
One time I had trouble with brakes and after changing many parts it turned out to be a faulty booster.

 

Dave,
Thought about a caliper(s) hanging up, but the "not full releasing" has only happened a couple of times in the past month...possibility for sure. Wheels don't feel excessively warm after a 40 mile drive. No vacuum leaks between intake and booster...check valve is operational.

 

Soft lines creating a check valve as they degrade

 

CSSB stainless steel braided brake hoses...no rubber. Thanks for the thought.

 

Wise man

 

Check the M/C fluid level . It could be suvked ito the diafram inside the booster .

 

Sounds like a faulty air admittance valve in the booster. It's supposed to close instantly when you take your foot off the brake pedal. That equalizes vacuum on both sides of the booster diaphragm so the diaphragm spring can release force on the push rod to the master cylinder.

If that valve didn't close your brakes wouldn't release at all.

 

I've got nothing, Leif! Just learning!!

 

Both bowls on the master cylinder are filled to spec...1/4" below top of rear deck.

 

Richard,
Is this "air admittance valve" something in addition to the vacuum check valve mounted on the exterior of the power booster? The vacuum check valve tests OK...air only enters from one direction. I follow what you're saying, though, as it feels like the vacuum assist is working intermittently during the initial part of my drives but seems to rectify the longer I drive...that's the perplexing part.
Keep coming with the ideas, guys.

 

Leif the booster should hold vacuum after you turn off the motor. Pull the check valve off, you should hear/feel air get sucked into the booster.

 

Leif:
Replace the failing booster. These have all sorts of failure modes. Yours is one of them.
I've got 9 antiques 8 with power brakes. I deal with this from time to time.

Dan

 

Dan,
The more responses I get and the more I analyze the situation, I'm inclined to agree with you. Fortunately, they're not very expensive and it's not much of a job. My wife has been asking what I want for Christmas, told her I don't need/want anything. Now, I've got an idea for her.
Thank you to all who have taken time to respond.
Happy Holidays to all of you!!