71 B Body power disc conversion pedal sinking

[cdkmbjones]

Good morning all, looking to probe the minds of the community. I have been working on cars and heavy trucks for 30+ years and recently came across an issue I need a little help with.
Did a power disc conversion on a 71 b body that came with 4 wheel HD 11" non power drum brakes. Bought one of the MBM "kits" wasn't thrilled with the look of the non OEM booster and off brand master and after installing it found my motor did not make enough vacuum either for the 8" dual diaphragm booster. The pedal was hard and stopping power terrible.
So, took it all back out and purchased an OEM 71 midland booster and all related bellcrank hardware from another 71 b body, had it rebuilt and restored. Installed it along with a reman 71 disc/drum master. Now my power brakes are beautiful but I can not get the pedal right. There is no air in the system - I have bled it with a power bleeder and a reverse bleeder - no air, nice clean streams of fluid. But the pedal goes right to floor. If i pump it, it builds a nice pedal and the car stops great. Wait three seconds and push again and it goes to the floor. I have adjusted the rod from the booster to the master right to the point of engagement. I have the rear shoes (with all new hardware, cylinders, drums, shoes) adjusted to the point of a slight drag. There are no leaks at any fitting and none of the rubber lines are expanding or collapsing - they are all new. as well. The front single piston calipers are new, with no leaks and after pumping the brakes, hold firm. The lines are all new, and the combination/ proportioning valve is new. After blowing my brains out on this, i've begun to guess - I never guess, but I am out of ideas and tests. I swapped out the proportioning valve - no change, swapped out the master - twice, once with a smaller 1.03 bore and once with a larger 1.15 bore and both times a new, not reman. The result is the same every time - the brakes work and the power assist is great, but it is like there is either air in the system - which there isn't, or like the pressure is not building and holding enough. I am open to ideas. I know there as been talk about bad proportioning valves out there made by inline tube, which is why I have not only used on of theirs but one from MBM - same result. It is more like the feel you get when the master is going bad - bypassing pressure internally.

 

[69Bee]

I'm no expert, but the only thing major I see you have only done once, is the rebuilt brake booster. You have had 3 master cylinders, two proportioning valves, and I would say calipers and hoses work or don't. The only other thing is the booster, check valve.

 

[cdkmbjones]

The booster check valve holds vacuum in the booster - vacuum isn't a problem - the "boost" is perfect. Its pedal travel that is an issue. I have to pump the pedal to build it up like when a master goes bad. Or when the mechanical linkage ratio is off and pressure has to be built to lessen the travel.

 

[YY1]

Is the prop valve for a disk/drum application?

Is it plumbed correctly for the residual valve side to be on the drum line?

 

[cdkmbjones]

Yes - it is the correct two component valve assembly for a 1971 b body disc/drum, all lines are in the correct location

 

[R/T Boy 67]

Like most problems, you have to start at one end a move toward the other. I would plug the master at both fittings. The pedal should be rock hard instantly. If it is, that eliminates the master and linkage. Then hook up one side and try again . Then do the other side while plugging the other one. That will determine which circuit is at fault.Then carry on down the line, plugging fittings etc., until the problem shows up.

 

[YY1]

Agree, but I'd elaborate by saying "plug one circuit, either front or back" rather than "one side". I'd start by plugging the back.

 

[cdkmbjones]

Funny thing is I had the same thinking and did just that - plugged both and have a rock solid pedal, plug one its better but still slowly sinks, reverse and plug the other - same thing. I bought one of those sets of plastic line clamps and clamped the rear flex line and no change, but if I also clamp one of the fronts it gets much better. I tried every combination of left, right and rear and when two were clamped it got better, all three and its rock hard. So, neither front nor back on its own made a difference but when you added in a third wheel it improved. This is the strangest thing. It seems like either a pressure or volume issue - which is why I went down the line of trying different bore size masters. Larger bore would increase volume, while smaller would increase pressure - still nothing different. This is probably something so stupid and ill feel like a fool but I have to admit I'm stumped. Every single part is brand new. I am going to try adjusting the booster rod a little more until it starts to hang up, I just feel like the issue is in the movement of the rod, that the rod isn't going deep enough into the bore so the pedal must be pumped to build up the pressure with smaller movements.

 

[walter funkhouser]

just a thought is push rod for mc adjustable from booster

 

[steve340]

It sounds like you have a faulty master cylinder. It is bypassing internally.
A loss of pedal pressure but no external leak is almost certainly master cylinder based on the info you have provided.
Replace with a new one. I have used 2 Hemi style units from the Ramman and they were perfect.

 

[R/T Boy 67]

Your master cyl is big enough. If the pedal is rock hard with all three of the hoses blocked, then the problem is down stream of them.

 

[steve340]

Even though you have tried different masters I still tend think master cylinder.
It is what generates the pressure. The only other thought I have is look for any other place the pressure can bypass from one circuit to the other. If something is bypassing you should see fluid moving in the reservoir when you apply pressure.
I had one on my GTX, brakes would be fine for 3 or 4 days and bam it would go to the floor.
Replaced the master all good.
Your problem is pretty strange. Good luck I hope you find it.

 

[cdkmbjones]

You guys are on the same track I was but I’ve replaced it not once or twice but three times and no change
Three different bendix masters correct for 71, two different bore sizes and one was brand new and no change.
It’s totally nuts. Was just out there now bleeding by pedal, by reverse pressure, by gravity - and no air yet the pedal will go to the floor but after two pumps will be perfect.

 

[5.7 hemi]

Post a pic and measurement of the linkage from the booster to the pedal. I had the same problem, turned out it was an E body linkage and was too short. The B body linkage is longer by about 3/8’s.

 

[YY1]

linkage is also different between power and non power

When the pedal goes to the floor, does the car stay stopped?

...or do the brakes actually release?