Brake Pedal Pressure Loss

[Ponch ®]

I recently started having an issue with the Satellite.

The brake pedal has been losing pressure whenever I'm stopped at red lights, stop lights, etc. Like, all the way to the floor with no resistance at all.

It doesn't happen all the time, only after its been driven for a few miles. The pressure slowly comes back once the car is moving again, but will drop again when I get to the next light and the car is at a full stop.

It may be relevant to mention that I've had some sort of voltage or vacuum issue where the car, at stop lights, etc. will suddenly almost shut off and then kick back up. I've had this problem since I've had the car but has not been a big enough deal, but the brake problem is recent.

Needless to say, the car hasn't gone and isn't going anywhere til I figure out this deal with the brakes.

Any ideas? Vacuum leak? Master Cylinder?

69 Satellite, front wheel discs.

[Back N Black]

Is there any visible leaks? is the master cylinder full? Sound like air in the system.

If there are no leaks it could be bad brake fluid. Brake fluid will absorb moisture and this can also cause spongy pedal problems. Water can get into the system through the rubber brake lines. It will lower the boiling point of the fluid so that, under heavy braking, the heat forms bubbles of gas in the fluid itself. This gas is compressible (whereas brake fluid is not); hence the spongy feel in the brakes.

Or
Brake drums that are woren past their maximum limit will cause the brake pedal to feel spongy. The spongy feel is caused by the drum expanding as the shoes press against it, either because of the crack or the thinness of the drum.

Also, could be master cylinder seals, fluid bypassing seals?

[John_Kunkel]

Intermittent loss of brake pedal height with your foot on the brakes is a master cylinder bypassing internally, replace it.

[Ponch ®]

Intermittent loss of brake pedal height with your foot on the brakes is a master cylinder bypassing internally, replace it.

that's what Im leaning towards. Gonna see if I can source the MC and replace it some time this weekend.

[ACUDANUT]

 

[Jaysick]

This happened to me in exactly the same way.  Replace the MC before they just go away entirely one day soon. 

[Ponch ®]

This happened to me in exactly the same way.  Replace the MC before they just go away entirely one day soon. 

that's the plan. Was gonna do it last weekend but Im waiting for a buddy whos more knowledgeable to give me a hand. Don't wanna "learn" with something as crucial as brakes.

[68charger440]

Based on your known issue with either vacuum or electrical issues, I would suspect a vacuum leak and go down that diagnostic road before replacing the master cyl.

The symptoms you describe... "It may be relevant to mention that I've had some sort of voltage or vacuum issue where the car, at stop lights, etc. will suddenly almost shut off and then kick back " point strongly to a possible vacuum problem. Inspect vac hoses for leaks and put a vacuum gauge on the car. Checking for leaks is easy and free compared with the cost and effort to replace the master cyl.

[A383Wing]

vacuum leak will cause very hard pedal, but it will stay solid....a bad master cylinder that is by-passing internally will have a symptom of pedal going to the floor without any resistance....I guarantee that you need a new master cylinder and you need to replace that first. Known electrical issue has nothing to do with hydraulic brake pressure

[ACUDANUT]

This happened to me in exactly the same way.  Replace the MC before they just go away entirely one day soon. 

Remember in 1964 they only had one reservoir. They are much more dangerous on the road should it fail.

[Ponch ®]

Based on your known issue with either vacuum or electrical issues, I would suspect a vacuum leak and go down that diagnostic road before replacing the master cyl.

The symptoms you describe... "It may be relevant to mention that I've had some sort of voltage or vacuum issue where the car, at stop lights, etc. will suddenly almost shut off and then kick back " point strongly to a possible vacuum problem. Inspect vac hoses for leaks and put a vacuum gauge on the car. Checking for leaks is easy and free compared with the cost and effort to replace the master cyl.

I mentioned the possible vacuum issue, but then again, the brake problem is recent and the vacuum deal has been forever. If they were related, wouldn't the brake problem have been there all along.

Either way, its going to my buddy's shop tomorrow (unless he flakes, which hes liable to do). Gonna try to drive it over, about 20 miles mostly freeway. First sign of trouble im pulling over and letting AAA handle it. its just faster and more convenient to drive it rather than wait around, etc.

He'll probably check for vacuum leaks anyway, so I'll keep you guys posted. Thanks for the pointers!

[Jaysick]

My booster just failed too (about a year after the MC) and it was a different feeling.  The MC failure made the brake mush, after working for a sec the peddle would all the sudden mush down to the floor.  When the booster failed I could still brake (although badly) I just had to really put my foot on it.  There was less peddle travel when the booster failed rather than it being "mushy".  Booster could still get one good brake if I went from high rpm to closed throttle braking from a freeway exit or something but the rest of the way through town I drove in first gear and left a lot of space between me and the car in front.  lucky it was really dead out and I didnt have any trouble with people getting in front of me.      Whew!

[Ponch ®]

Got the car over to my friend's shop, uneventfully. The rear reservoir was empty (checked it before driving out), refilled it, and had no problems on the way over.

The car's still over there since he doesn't really work on weekends, but after he spent some time looking at it, the preliminary diagnosis is that I have a bad wheel cylinder in the rear drums. They'll get replaced and then move on to see if the MC is still causing problems or not.

[John_Kunkel]

A leaking wheel cylinder would show by a puddle on the ground. More likely the fluid in the rear reservoir leaked into the booster. (leaking piston seal)