Mystery brake issue.

[celticwarlock]

1969 NY, front disc / rear drum.

A while back, I replaced the rear brake hose which was restricted due to age. Not long after, one wheel cylinder started leaking, so I replaced them both (they were severely corroded on the inside).

What was weird is that the lines bled, but they bled slowly, and the pedal never went to the floor. I had the bleeder screws turned out way more than what's typically required, but still, the pedal was way more stiff than it should've been. The fluid that came out was a bit dark at first, but then got nice and clear.

The car's brakes operate quite well. They don't pull to one side, fade, or do anything goofy. The front calipers and hoses were replaced some years back, so I can't see them being the issue. In any case, nothing up front should affect how the rear lines bleed.

Yes, everything works, but this isn't normal, and I find it unsettling. Any ideas what could be causing this?

 

[Rusty Muffler]

My first thought would be to remove a bleeder completely and see if it's partially restricted. Also look in the hole too. The crud and corrosion is the clue.

 

[celticwarlock]

My first thought would be to remove a bleeder completely and see if it's partially restricted. Also look in the hole too. The crud and corrosion is the clue.

The wheel cylinders were replaced. Upon removal, the lines were dripping, so I can't say there's a restriction further up the line. They were dripping pretty slowly, but dripping nevertheless.

 

[mikedrini]

Have you checked the distribution block or proportioning valve, depending on which one you have? Could be partially plugged or maybe the proportioning valve needs to be reset?

 

[halifaxhops]

I would also check the rubber line going to the rear.

 

[Just Carbs]

Remove one of the steel lines connected to one of the rear brake cylinders and pump the brake pedal.

 

[celticwarlock]

I would also check the rubber line going to the rear.

I slready replaced it a couple of weeks ago. I mentioned this in my initial post.

 

[celticwarlock]

Remove one of the steel lines connected to one of the rear brake cylinders and pump the brake pedal.

I'll try this, although I can't imagine things would be different from turning the bleeder out several turns. I'll give it a shot snyway.

 

[celticwarlock]

Have you checked the distribution block or proportioning valve, depending on which one you have? Could be partially plugged or maybe the proportioning valve needs to be reset?

I considered this a possibility. Can I use compressed air to clear it, if it has a blockage?

 

[mikedrini]

I considered this a possibility. Can I use compressed air to clear it, if it has a blockage?

You could try? But if you have a distribution block, I would say remove it and flush it with some brakleen. If you have a proportioning valve, there is a reset tool you can buy for a few bucks or you can use a small screwdriver. Not sure what you have, but you're most likely looking at having to bleed the brakes once again.

 

[celticwarlock]

What's the difference between the 2? Is one merely a splitter (distribution block), and the other plays an active role in regulating the hydraulics (proportioning valve)?

I thought every vehicle built after a certain year has a proportioning valve.

 

[68PK21 440.6bbl]

Could be some blockage in the rear brake part of your warning light switch.

Also what kind of quality of rear brake rubber hose did you get? Faulty manufacture of new hoses can happen.

.

 

[celticwarlock]

The rubber line is a Raybestos part.

 

[celticwarlock]

Alright.

I disconnected the main line from the brake hose. I then pressed on the pedal, and still, it wouldn't go to the floor. It did, however, move fluid through the line.

I then disconnected the line at the master cylinder and ran compressed air through the entire length. There was no obstruction. I also ran air through the line cavity into the master cylinder (gently) and again, there was no clog.

I reassembled everything and successfully bled the system. Again, everything works, but damn, this is weird.

 

[3C's & a D?]

So everything is working properly, your concern is the pedal not completely going to the floor? And seemingly less fluid motion to the rear?

Was the master cylinder or power booster changed? Sounds like the pushrod may not be properly adjusted. You could try loosening the nuts that hold the master to the booster, and pull the master forward a quarter inch or so, then try the pedal. If you get more pedal travel, that would indicate pushrod adjustment or possibly incorrect master cylinder.

 

[68PK21 440.6bbl]

So everything is working properly, your concern is the pedal not completely going to the floor? And seemingly less fluid motion to the rear?

Was the master cylinder or power booster changed? Sounds like the pushrod may not be properly adjusted. You could try loosening the nuts that hold the master to the booster, and pull the master forward a quarter inch or so, then try the pedal. If you get more pedal travel, that would indicate pushrod adjustment or possibly incorrect master cylinder.

I think a without pictures this post is useless meme is needed here, he may have one of these chineseium colored boosters with a GM master cylinder setup. Kind of a mystery that a couple of pictures could solve.

.

 

[celticwarlock]

So everything is working properly, your concern is the pedal not completely going to the floor? And seemingly less fluid motion to the rear?

Was the master cylinder or power booster changed? Sounds like the pushrod may not be properly adjusted. You could try loosening the nuts that hold the master to the booster, and pull the master forward a quarter inch or so, then try the pedal. If you get more pedal travel, that would indicate pushrod adjustment or possibly incorrect master cylinder.

Nothing has been changed other than what i mentioned.

What's got me concerned is that opening the system at any point should allow the pedal to go to the floor, and with this car, that isn't happening. Disconnecting the main line before the split at the differential, disconnecting the lines at the wheel cylinders, or opening the bleeder screws wide should allow the pedal to go down to the floor. With this car, that doesn't happen.

 

[Just Carbs]

Nothing has been changed other than what i mentioned.

What's got me concerned is that opening the system at any point should allow the pedal to go to the floor, and with this car, that isn't happening. Disconnecting the main line before the split at the differential, disconnecting the lines at the wheel cylinders, or opening the bleeder screws wide should allow the pedal to go down to the floor. With this car, that doesn't happen.

What do you know about the little pressure retainer valves in the master cylinder?

 

[celticwarlock]

What do you know about the little pressure retainer valves in the master cylinder?

Absolutely nothing. I assume they're internal....

The reason for this whole post is that this isn't the first time I've worked on this brake system. I've been all over this car. I've owned it for nearly 20 years. In every single case, without exception, I have never seen this when working on the system (replacing parts, etc.).

 

[Just Carbs]

Absolutely nothing. I assume they're internal....

Yes. At the connection point where the brake lines connect to the master cylinder.
They pass pressure out unrestricted but hold 2 to 10 psi from going back in, to keep the shoes from being pulled away from the drum by the shoe springs when you let up on the brakes.
The valves are supposed to be matched to the shoe spring power. The idea was to keep the shoe as close to the drum as possible without the shoes dragging on the drum.
You can't see it because it's hidden just behind the line seating surface/cone on the master cylinder. It's held captive in place by the brass seating plug the line seals to. It's a little spring and plastic valve.
If that crappy little valve has messed up it might be reducing line fluid flow in some way.
To remove it, thread a tap (just far enough) into the brass line sealing plug, pull the brass line sealing plug out. Then pull the valve out if it doesn't fall out.
Inspect valve as best can be. Reinstall brass sealing plug. With or without valve.