Brake issue on the '74. Maybe combo/proportioning valve?

Figured I'd ask about this, even though this car isn't super advanced to the level of the 90s B's, I think much of the parts are the same, and it's a basic braking system.

A few weeks ago when driving the 74 Impala, as I was coming to a stop, I noticed that the brake pedal got hard while the stopping power seemed a little weaker. All of a sudden I felt a pop sensation through the pedal, and the warning light came on, and stayed on. However, the brakes then started working better. Stayed on until I parked the car, probably drove it for a half hour or more like that. When I started it again, it was on, so I pushed down hard on the brake pedal as you would after doing a brake line repair or something to "reset" the brake warning light. It popped again, and the light turned off.

So then it happened again when I drove it, and it did it again today. The pedal doesn't go to the floor or anything like that, so I don't see how I could have lost fluid or anything at all. It still stops good with the warning light on.

The car has a new master cylinder and brake booster (well, a year and a half ago, and reman parts...). I will check on Tuesday or something but I don't think it's lost any fluid.

I'm pissed about this because the sticker just expired apparently so now I'll have to get it fixed before I go for a new one I guess because the brake light being on just doesn't look good as far as I'm concered, it's not like a CEL...

Does this sound like a bad proportioning valve to anyone? I don't know what happens when one of them goes, but I do know that they somehow sense when there is a pressure differential between the front and back brake systems or whatever, and that triggers the light on the dash (like if you blow a wheel cylinder and lose fluid, etc.). As far as I know, the brake warning light is brake pressure/fluid related ONLY, aside from going on when you set the e-brake.
I'll have a better idea when I take the cover off the M/C and have a look. I also don't get how the vacuum booster could be related, because how could it make my warning light go on?

I just figure if the fluid leaked or I had air in the lines, the pedal would feel like crap. This car has a "good pedal", so to speak.

Friggin' cahs! I need to downsize.

This is your chance to convert to Disc brake

It already has disc brakes!

The warning light on the prop valve is just "balanced" between 2 pressurized systems (front & back)
If 1 side loses pressure, the valve slides towards that side shutting it down & turning on the warning light

This popping / on / off thing is kinda funky
When was the last time you cracked the lines open?
If ..never....I'd question where air could get in the line....but that would make the valve shift..

Matt Trakker wrote:

It already has disc brakes!

Rear Disc ?

My first guess would be air in the system, but like Steve said, from where.

Brake light only comes on when proportioning valve moves from one side to the other. Period.
When you did the master cylinder did you bench bleed it first?
You could have one side of the MC failing and causing the proportioning valve to slide. No external leaks would happen and you wouldn't lose fluid. Check the master cylinder for bubbles while someone steps on the brakes with car on.

Wow, thank you for all the replies so quickly, I didn't expect this many, lol.

No it has rear drum, Tom.

Yeah, what happens is I'll be driving, and the pedal suddenly pops up ever so slightly and gets firm, while the brakes themselves feel weak. Then suddenly the "pop" sensation and the warning light goes on. This same pop is the one that happens when you "reset" the combination valve after doing brake work to make the light go off. But after the pop/light, the brakes work better.

The pedal does seem kind of high, and the brakes, while not terrible, seem like they leave something to be desired in stopping power. As long as I've had this car, also, the rear brakes can easily be locked up by stepping on the service brake. They've always been very touchy, but it didn't bother me.

I have had brake work done on the car since I've owned it. When I bought it, it had a blown rear brake line. I had that replaced, and the brakes were bled I'm assuming, but I wasn't there. Then a few years later, I had a rear line go again, and a section was spliced in.

A few years ago the booster was hissing and I think making a vacuum leak as well, so I had that replaced, and the master as well because it was going to be off the car, and I was starting to get that problem where the pedal will drop a little every five seconds of constant pressure on the pedal, like when you're stopped at a light. I'm fairly certain the master was bench bled. I'm going to talk to the guy that did it because oddly enough he is fixing my lawnmower this week. He's pretty good and I'd be surprised if he did not do that. I had some other guy work on a truck I had years ago after a friend told me to go there, and he did my M/C and calipers and didn't even bleed my rear brakes, and I don't think he bench bled the master either!

I suppose it's possible that there's just air in the system from one of the three times the lines have been open since I've had the car. Also, before I bought it, this car sat untouched for about the entire 1990s. Someone resurrected it (it had 29000 miles on it in like 1999) and replaced pretty much everything like calipers, hoses, etc. I don't have proof but the calipers definitely don't seem to have rusted 40 year old bleeders, etc. The rear drums do still have stickers from the factory on them though, and those keepers that go on the lug studs.

Indy, do you mean to look at the M/C externally for bubbles, or with the cover off and watch the fluid? I'm definitely naive here and always figured that if air were in the lines, the pedal would be spongy as hell.

I don't know if I'll make it to Malden today to check this out but definitely tomorrow.

Thanks guys. The thing that drives me crazy the most is that it drives fine and still stops pretty good considering. The warning light is tormenting me. I was also thing of taking this on a Sunday out to the hairpin turn on Route 2 and then hitting a cruise night along the way on the drive back, but now I don't want to until this is fixed lol.

Yes...the Warning (idiot) light is your FRIEND....

Air in line does feel spongy, what I'm saying is the fluid could be leaking between the front and rear reservoirs in the MC, hence you wouldn't lose any fluid nor gain any air. The lack of pressure FROM the master to the porportioning valve on whatever side is leaking might cause it to read as a "leak" and slide forward/backward. As you let off the pedal the fluid pressure equalizes in the MC, you pressing the pedal hard resets the valve and the light goes out. the pop might be fluid rushing to one side of the MC, the pedal gets "good" because the proportioning valve closes off the side in the MC that's leaking there by giving you only the good working side of the brake system,
If you have a plastic MC look with the cover on with a flashlight shining through the side. If it's a metal one you can look with the top off, BUT you have to press the brakes in slow or you'll blow brake fluid everywhere.

A simple rebuilt kit might fix the problem if you don't wanna buy another MC.

Ironfistdog wrote:

Air in line does feel spongy, what I'm saying is the fluid could be leaking between the front and rear reservoirs in the MC, hence you wouldn't lose any fluid nor gain any air. The lack of pressure FROM the master to the porportioning valve on whatever side is leaking might cause it to read as a "leak" and slide forward/backward. As you let off the pedal the fluid pressure equalizes in the MC, you pressing the pedal hard resets the valve and the light goes out. the pop might be fluid rushing to one side of the MC, the pedal gets "good" because the proportioning valve closes off the side in the MC that's leaking there by giving you only the good working side of the brake system,
If you have a plastic MC look with the cover on with a flashlight shining through the side. If it's a metal one you can look with the top off, BUT you have to press the brakes in slow or you'll blow brake fluid everywhere.

A simple rebuilt kit might fix the problem if you don't wanna buy another MC.

Oh, good to know then!

The car has a cast iron MC. I believe it was a Wagner (prob. made in China). I did save the original factory one though, (it still had the stickers and stuff on it) just in case.

So I'm assuming it would be bubbles of any sort emanating from down inside the reservoir? I have to say, the scenario you outlined there kind of sounds like what it might be doing...

When driving, it sort of feels like the brakes are working "enough", in a way that maybe it's the rear portion of the brakes that gets closed off and they're slightly weaker but the pedal is still good.

Soooooo...I just checked the MC on the old Impala. The smaller reservoir for the rear brakes is completely empty.

I moved the car right before I checked this, and when jockeying it around I noticed the brake light did not go off again after I pushed the brakes hard. So I'm assuming that this was a sudden but relatively slow leak as opposed to a burst brake line or whatever.

I don't know if this means my MC is okay since that reservoir is all drained out...I mean where could the fluid have gone? I did not get to look hard but did not see any obvious places. I'll fill it tomorrow and see if I can see anything going down the rear tires or something. Maybe a wheel cylinder blew...

The weirdest part is that the pedal still feels pretty good.

Roll the girl in flour and look for the wet spot . Get her up on the lift and check her out, maybe along the the frame? got to be going somewhere.