Soft brakes after rear disc install

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Agent_Orange

Agent_Orange

Well-Known Member
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Location
Indiana
1974 Duster
318 15lbs vacuum
Power brakes
MC front disc rear drum
Factory front rotors
8.8 Ford rotors in back

the car stops but pedal is near the floor before I see any stopping power. Is this a simple MC swap or will I need more gear?
 
There is a residual valve in the original master for the rear drum brakes. Usually comes out on a disc conversion............Maybe something in the proportioning valve? Is all the air out?
 
Agent_Orange said:
1974 Duster
318 15lbs vacuum
Power brakes
MC front disc rear drum
Factory front rotors
8.8 Ford rotors in back

the car stops but pedal is near the floor before I see any stopping power. Is this a simple MC swap or will I need more gear?
Click to expand...
You really need a MC that is for dual disc brakes. Note the reservoir on the OE MC has a larger reservoir on the back for the front discs. There is a reason for that.
 
Ricks70Duster340 said:
You really need a MC that is for dual disc brakes. Note the reservoir on the OE MC has a larger reservoir on the back for the front discs. There is a reason for that.
Click to expand...
That was my thought too. I need a MC with equal large reservoirs to compensate for brake pad wear. Any recommendations on what MC to source?
 
Agent_Orange said:
That was my thought too. I need a MC with equal large reservoirs to compensate for brake pad wear. Any recommendations on what MC to source?
Click to expand...
Some of the others who have done this upgrade may chime in. I still (so far) run rear drums so use the OE style MC.
 
Ricks70Duster340 said:
You really need a MC that is for dual disc brakes. Note the reservoir on the OE MC has a larger reservoir on the back for the front discs. There is a reason for that.
Click to expand...
No.

Only for pad wear. Not for function in the immediate sense

The reason for the big "tank" is to be sure the thing stays full with pad wear. I have 73/4 Duster Demon front disks, with Lincoln Versailles (Ford) 9" factory rear disks, and am using THE FACTORY DRUM master on my 67, which was 9" brakes and in my case, no prop valve. I simply punctured the residual valves

It gave me above 1/2 pedal when "down," hard as a rock, would rattle the tire treads on the freeway at 70
 
Agent_Orange said:
What's the procedure for puncturing the residual valves?
Click to expand...
LOL Bear in mind this was done "to see if the master would work" You are supposed to remove them. The shop manuals all have a diagram, sometimes using a sheet metal screw and claw hammer
 
Did you check / did anyone ask are you SURE the bleeders are on the top most side of the calipers, AKA switch them side for side? It' is always possible the master is not properly bled and that it is defective. Otherwise, air in the system. Try buying some inverted flare plugs and unions so you can cap off sections, starting with the rear tube where it meet the rear hose.
 
The symptom you are getting is not of using a res line pressure valve. Using a RLPV with disc brakes causes the pads to drag & a HIGH pedal, not a low pedal.
Sounds like air in the lines.
 
Bewy said:
The symptom you are getting is not of using a res line pressure valve. Using a RLPV with disc brakes causes the pads to drag & a HIGH pedal, not a low pedal.
Sounds like air in the lines.
Click to expand...
There is air in the line. I started bleeding last night and found out I'm out of fluid
 
Besides air, you might have lost-motion between booster output rod and MC piston. Most boosters have an adjustable tip. You want to take out all play, but not compress the piston (much) when bolting them together. If you do, the piston may block the fill port. That causes the hydraulics to lock up after driving a bit and the fluid heats and expands but can't relieve pressure thru the fill port, so the brakes quickly overheat and smoke (how would I know?). I bought a $15 tool on ebay to adjust the output rod.

Re return of disk pads, they don't use a spring, as in drum brakes, so there is no gap adjustment. What returns the pistons is that the seals are square rubber and rotate when the piston extends then rotate back to pull the piston back. Sounds quirky, but seems to work well as long as a dust cover keeps the piston surfaces pristine. The piston seals must be a good rubber since lasts surprisingly long (Viton?).
 
So I replaced my disk/drum proportioning block with a drum/drum distribution block last night, and my disk/disk master cylinder will be in on Monday. I also picked up this adjustable proportioning valve from piratebay.

My question is, with having 1 front brake line feeding the block, am I alright to plug one of the outlets in the adj valve?

20230429_113111.jpg
 
I think you mean that only 1 front tube will exit the proportioning valve, in which case, you should block the other front output port (FO). Autozone carries SAE inverted flare plugs in bubble packs (or did). Presumably, a downstream tee goes to both front brakes. Perhaps what you mean by "drum-drum distribution block". The rear tubing shouldn't go to that block, and only connect to the "RO" port.

The switch at top of photo is a "pressure imbalance switch" which will light the "brake" dash lamp (none in my early-A's but added a lamp). It grounds to light the lamp, which you can parallel to other brake switches which ground as a warning (e-brake on, MC reservoir low). If the imbalance switch trips with no pressure on the brake pedal, you might need to crack some ports to relieve pressure to re-center it. At least I've read that for the factory switch, though seems unnecessary since it should measure FI and RI pressures which should be open to the reservoir (fill ports open) with MC piston retracted, but someone might explain why not.
 
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BillGrissom said:
I think you mean that only 1 front tube will exit the proportioning valve, in which case, you should block the other front output port (FO). Autozone carries SAE inverted flare plugs in bubble packs (or did). Presumably, a downstream tee goes to both front brakes. Perhaps what you mean by "drum-drum distribution block". The rear
Click to expand...
yeah, the brass distribution block was used in drum/drum cars. Factory cars with disc front and rear drums had a slightly larger brass block that was referred to as a proportioning block. From my understanding, the drum/drum block is, essentially, a straight through T fitting for the front circuit and a coupling for the rear circuit. Both the distribution and proportioning block have the same line placement. Front circuit in the top of block with 2 lines going feeding the front brakes. And 1 rear brake circuit feeding the one brake line heading to the rear.

I would like to place my adjustable proportion valve between the master cylinder and the drum/drum distribution block. This exact unit has the option to directly feed both front brakes which I don't want to do. My question for the group was if there would be any problems with plugging one of the outgoing front circuit ports and sending the other to the incoming port on my distribution block.
 
Your plan will work. I also kept the distribution block in my 1965 drum-drum car since that let me use the existing front tubing. It was just a 4-port "tee" in 1965 drum cars, with single-pot MC with 1/4" tube to the top of the block. I put a 3/16" inv flare adapter at top port for the F tube from dual-pot MC and plugged the rear port. The R tube from MC goes to the existing rear tube via an inv flare coupler. I also installed an adjustable proportioning valve in the rear tubing in case I later change to front discs. Might have been better to have used the one you did to get the pressure-imbalance switch. But, often that switch didn't work in OE systems since got stuck by rust. Since I've used silicone brake fluid in my vehicles I no longer have internal corrosion issues.
 
Have everything installed. Master cylinder was bench bled. And I cannot get this rear circuit to bleed. So how long should it take to power bleed an empty line? I havent used a pneumatic bleeder before. I let it run for about 15 minutes. If that's normal then okay, but I'm thinking there has to be a leak somewhere
 
Sorry for my ignorance, but I want feel safe picking my kids up from school in thos car. When I removed the old m/c, the metal bracket in the pic below was sandwiched between the m/c and booster. Was I suppose to reinstall that bracket, with a gasket, between the m/c 2 hole adapter and booster?

I ask because of an observation I made the second time installing the m/c.... (bench bleed hoses were still attached to m/c ports and pinched off) M/C slid onto booster studs until it was about 1/4 of an inch from the booster. I pushed the master the last 1/4 in. And pressure popped the line off the rear brake port. Luckily I had towels covering everything. I didn't pay it much attention until I went to bleed the rears. Wouldn't that 1/4 inch of stroke in the master close the rear circuit off to bleeding?

Sorry for the novel guys. I've exhausted the search tool on here for a week and a half. Trying everything that felt logical.

We're lucky to have such a helpful community

20230507_210114.jpg


20230507_210032.jpg
 
I don't mess with power brakes at all on old Mopars.....but I think your issue may be the piston in the MC is starting too far in. You are not uncovering the rear port to feed the fluid into the rear piston area.
Is there an adjustment somewhere to shorten the pushrod?

That 1/4" metal piece may be the clue.
 
This seems to be a common theme on rear disc conversions. Especially putting together a non component matching brake setup.
There is really no reason to replace the rear drum brakes. The front do the majority of the stopping. Need more stopping power ? Use more aggressive rotors, pads and even dual/sided piston calipers.
The brakes are the most important safety system on your car, especially for driving on the street.
The main reason that most modern cars come with 4 wheel disc is more for economic reasons than safety.
At the factory/assembly line, disc brakes are cheaper and require no adjustments on installation.
Most have ABS pumps to give high line pressures and electronics to constantly adjust individual wheel braking .
Not so on our older cars.
It would take thousands of dollars to duplicate the brake systems for the old cars.
But most everyone tries to go the economy route mixing parts from different manufacturers and than wonder why there are problems.
Try mixing in Ford or GM parts when building a Mopar transmission or engine and see what happens.
Stick to rear drum and your problems will go away and your car will stop great again, or just upgrade component quality.
 
KosmicKuda said:
I don't mess with power brakes at all on old Mopars.....but I think your issue may be the piston in the MC is starting too far in. You are not uncovering the rear port to feed the fluid into the rear piston area.
Is there an adjustment somewhere to shorten the pushrod?

That 1/4" metal piece may be the clue.
Click to expand...
thanks for replying, as I ment to type up a conclusion for anyone in the future who stumbled onto this thread.

The problem was 3/8" of preload on the m/c. I removed the pushrod from my booster, removed the threaded acorn bolt, cut 3/8 off the end, ran a 1/4-28 tap to clean up the threads, and reassembled. No more preload on the m/c and perfect pedal height. Brakes took a couple bleeding sessions to get all the air out but they are now working great and I have no brake safety light when fully applied.
 
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