71 Duster front disc brake conversion

[bahamabecomb]

I am unable to figure out why the two brake caliber pistons are not releasing fully once the brake pedal is left off. Front brakes are holding after a stop for about 3 seconds and then then partially retract. When I put the car on a lift, press the brake pedal and release both front wheels are still difficult to turn by hand, drivers side loosens up after about 6 seconds but passenger side still takes some force to rotate. I took the calipers off, opened the bleeder valves and compressed the pistons back but the problem starts over again. Also, from the pictures attached which is the correct way to mount the two pieces of the brake caliper bolt retainer and clip? One side had the smaller clip underneath the main clip and the other had it on top of the main clip. I'm assuming this is for the wear indicator as the pads have no markers on them.

 

[RustyRatRod]

Unbolt the master cylinder from the firewall or booster and see if the calipers unlock. If so, you need to shorten the pushrod.

 

[Professor Fate]

What master cylinder do you have?
If you have a drum/drum master cylinder, then the residual valve in the front brake circuit could be holding up your calipers- they should only be used in drum brake circuits. Remove the residual valve or swap in the proper disc/drum master (it also has a larger capacity reservoir for the front calipers).
If you have a disc/drum master, do you have it plumbed properly? The lines for the front brakes are plumbed to the rear reservoir. If it is plumbed to the front reservoir, you will be dealing with the residual valve meant for the rear drums. Re plumb the lines correctly.
Lastly, double check your line routing going into and out of the proportioning/combination valve.

 

[bahamabecomb]

Here is picture of the master cylinder. I believe it is correct for front discs/ rear drum. I will check the plumbing of the proportioning valve when I get back this afternoon

 

[4mulas]

Just as an aside. You can’t just crack the bleeder and compress the piston back and just close the bleeder. You’ll have to re-bleed the system.

As mentioned, also start by Checking the push rod length, then also see if there is a residual pressure valve in the front line circuit at the master

 

[bahamabecomb]

Thanks for the reminder on bleeding the brakes, there was a lot of air in there. Checked the plumbing to the proportioning valve and everything looks correct according to the diagram. Took it for a drive and it is noticeably better, brakes hold when I take my foot off the pedal but after about one tire rotation they retract. Not sure if that is a normal range for a lot but I will look into the push rod tomorrow, what type of length am I looking for? Thanks

 

[72bluNblu]

All good advice on things to check so far.

Can you actually feel the brakes dragging when you’re driving?

One thing to keep in mind is that disk brakes don’t have any mechanism to retract the calipers. The pads are knocked back by the movement of the rotor only.

So with your example with the car on a lift, the wheels would absolutely remain difficult to turn by hand for a couple revolutions before the pads get knocked back after a brake application, that’s completely normal. While driving that happens pretty much immediately, otherwise you feel the brakes dragging.

 

[Professor Fate]

Thanks for the reminder on bleeding the brakes, there was a lot of air in there. Checked the plumbing to the proportioning valve and everything looks correct according to the diagram. Took it for a drive and it is noticeably better, brakes hold when I take my foot off the pedal but after about one tire rotation they retract. Not sure if that is a normal range for a lot but I will look into the push rod tomorrow, what type of length am I looking for? Thanks

A body power brake systems do not use a conventional brake pedal pushrod that lends itself to any kind of adjustability- it is a solid link.

The only part with any adjustability is the rod coming out of the booster, and the master must be removed to access it. There is a small acorn nut on the end of the rod that allows a small amount of adjustment, but the setting must be precise. The details and measurements are in the FSM.

If additional adjustment is necessary, it's cut and weld time on the pedal linkage arm, as there is no aftermarket adjustable piece like there is for manual brakes, at least to my knowledge, for factory boosters.
That said, you appear to have an aftermarket (Dr. Diff?) booster- the booster rod adjustments should be the same, but many aftermarket booster kits come with a couple different length pedal linkages- if your kit came with any, you MAY be able to use one of those if necessary.

 

[bahamabecomb]

Left front has been exactly as you described, right front really was dragging until I re-bled so I think it may have been air in the line? Lots of bubbles came out. I'm going to check my other road runner which had a similar disc conversion and see if it does the same.
Thanks

 

[JDMopar]

Like RustyRatRod & Professor Fate suggested, take the master cylinder loose from the booster. Leave the bail clipped on the cap and you probably won't lose any fluid. Measure from the flat back of the master cylinder surface that meets the booster, to the inside of the little cup in the end of the plunger held in with a snap ring. Now measure to from the surface of the booster that the master cylinder sits against, to the tip of the rod that pushed into the aforementioned cup in the master cylinder. If that measurement is greater than the other one, you should be able to use 2 little wrenches to shorten that rod up to match. It just need to touch the cup...not push on it without pedal movement.

 

[RustyRatRod]

A body power brake systems do not use a conventional brake pedal pushrod that lends itself to any kind of adjustability- it is a solid link.
View attachment 1716386247
The only part with any adjustability is the rod coming out of the booster, and the master must be removed to access it. There is a small acorn nut on the end of the rod that allows a small amount of adjustment, but the setting must be precise. The details and measurements are in the FSM.
View attachment 1716386252
If additional adjustment is necessary, it's cut and weld time on the pedal linkage arm, as there is no aftermarket adjustable piece like there is for manual brakes, at least to my knowledge, for factory boosters.
That said, you appear to have an aftermarket (Dr. Diff?) booster- the booster rod adjustments should be the same, but many aftermarket booster kits come with a couple different length pedal linkages- if your kit came with any, you MAY be able to use one of those if necessary.

That's the rod I was talking about if it has power brakes. Finally on post #4, we find out it has power brakes. Four nuts and less than five minutes to find out if the pushrod needs shortening. Good GAWD almighty.

 

[72bluNblu]

If the left and right are different it’s not the booster.

 

[bahamabecomb]

I'll get the master cylinder off in the next day or so and take a look. Seems like 72bluNblu has a good point though. Passenger side piston was staying 1/8" out compared to drivers side but they are returning to the same place now.