Disc Brake conversion issue

-
Rockerdude

Rockerdude

Rock n' Roll and A-bodies
FABO Gold Member
Joined
Aug 10, 2007
Messages
2,497
Reaction score
167
Location
Arizona
Hey guys I put a set of bbp Discs on my wagon and I have been trying to figure out an issue that I've been having.

I replaced the calipers, rubber lines, master cylinder (was previously upgraded to a dual stage one before I got it) banjo bolts, rotors were turned, and new wheel bearings too.

When I go to bleed the fronts, you pump the pedal a ton to build a tiny bit of pressure and when you release the bleeder not much comes out and you have to pump it up a ton again. Then I would go for a spin around the block and the pedal gets super firm and the front calipers start dragging super hard to where you cant spin the wheel if it were jacked up off the ground. If I leave it over night, they release. It actually first did this with the old master and calipers, so I replaced the calipers first and when that didn't fix it I replaced the master too.

Still having that issue and I'm thinking it might be the new banjo bolts.. Here is a picture of a new one from Orielly's and an original from a 75 Dart. The original is on the left, see any issues? I measured the 'neck' where the hole is and the original is slightly smaller at .397, the new one was .405. Also looks like the factory ones have a tapered hole and a larger inner diameter too...
 

Attachments

  • 1977279_595544523911357_848596935052833938_n.jpg
    56.9 KB · Views: 662
It could be the wrong master with a check valve in it. Do you know what the master number is or what it was ordered for? Did you bench bleed the master before hooking it up to bleed the system?
 
Its a master cylinder for an 87 dodge truck with the small piston, I did bench bleed it very thoroughly. The strange thing is that it still did it with the old dual master cylinder that was off of a 72 Dart. Here is a pic of the new master.
 

Attachments

  • 20141201_173835.jpg
    86.1 KB · Views: 622
It sounds like it's plumbed backwards,, residual valve holding pressure in the fronts,, the rear M/C brake line should go to the front brakes..
 
No, I checked that and its plumbed correctly..
 
This is so simple. The compensating port is not working.
It may just be that the pushrod needs to be adjusted. I think in your case a tad shorter would do it.
Its also possible that the pedal is not parking properly, so that the port doesnt get exposed.
For some reason the fluid is not returning to the reservoir.
 
The rod is not adjustable, I know its simple but its really getting me scratching my head.
 
You could always get to where it's got the pressure built up, where the wheels drag, and remove the bolt where the pushrod attaches to the pedal and see if it releases the pressure. If so you may have to make or buy the adjustable push rod.
 
I am going to give that a shot as well as change the banjo bolts to original style ones.. Thanks
 
I had the same problem on a 93 Toyota truck yes it will give you fits I sold the truck and never fixed the problem but I think I would check into the residual valve idea. Good luck
 
Remove the brake light switch and see if the pedal goes out further. If so, that was likely the problem. Then, re-install the brake switch so it just senses but doesn't interfere with MC movement.
 
Does it do it on both front wheels? I had one do exactly as you describe but only on one wheel. Was a bad rubber brake line (a bad new one at that).
 
It has a distribution block, no proportioning valve. It was working just fine with the 73 drums it used to have.
 
You replaced the master ....did you check to make sure there is a little free play between the pushrod and the plunger in the m/c ?
 
If the distribution block has a warning light switch then another thing to check is the shuttle pin inside, is it centered? Not uncommon for bleeding to move the pin and block off or mostly block off either the front or the rear part of the system.
 
Yes there was play between the plunger and rod. I will take another look at the prop valve.
 
Ok then, Ill try to walk you through it.But before we start reread post#17
As the pads and/or shoes wear out and get thinner,the pistons that operate them move out of their bores.This happens on a continuous, daily basis. Every time you use the brakes, a bit of material sacrifices itself to heat and the material gets thinner.And the pistons move out. Of course when the pistons move out there is a tiny bit of fluid that doesnt return to the M/C. To COMPENSATE for this there is a tiny hole in the bottom of the M/C fluid chamber. So everytime your pedal parks at the top of its stroke, this little port is exposed to the fluid and it flows into the piston chamber. Then when next you step on the brakes, the first thing that happens is the piston passes by that hole and shuts it off. Then the braking begins.
-Try to imagine what would happen if this port was never exposed to the fluid. All the fluid in the piston chamber would eventually move out to the calipers/wheel cylinders, and the pedal would drop and drop, until eventually there would be no braking power at all.
-Next, try to imagine what would happen if the fluid never all went back to the M/C after each application. So you step on it and a bit of fluid goes out. Then you release and not all comes back. If you do this several times you will eventually push the pads out hard against the rotors. The pedal will get rock hard and the brakes will be stuck full on. Sound familiar?
-So whats the issue here?
Part 1;The fluid is not returning, or not returning in a timely fashion, since you say, overnight it releases the pressure.
Part 2;Youre having difficulty bleeding it.
Part 3;Everything is new or reconditioned. Well almost everything.The hard lines and any valves on it, are not new.
Part 4;It did this with all the old parts
Heres where it gets interesting. You said the calipers stick on. Thats plural.That tells me the problem is not 1 new hose, and not 1 hardline to either wheel. Odds are very good the problem is not there.Lets move ahead.Whats between the junction block where the downline splits to the two wheels,and the M/C? I believe there is a restriction in that section, almost to the point of plugged. The hardline being so high in the system, is not likely to be faulty unless its been pinched. ie pliered or hammered. Inspect it carefully for signs of such damage.So what does that leave us with? Well just the J-block.
-Now,depending on what we are working on,That block could be;A) a J-block,or b) a Proportioning Valve, or C) a metering Valve, or D) a brake warning lamp, or E) some combination of valves, never more than two. There could be one or two valves there. I believe, as others here have said. that your problem lies here.
-Now getting back to the C-port. The fastest way to prove a restriction exists is to check the port for return flow. Now in your case, we already know the fluid is not returning, because the brakes stick on.Or do we? Its possible for brakes to stick on for other mechanical reasons; thus the C-port test. You will need a helper.If you have anything other than Dot5 in the system, and even then, you will have to protect your paint from spilled fluid. So take a moment to cover everything around the M/C within about an 18 inch radius.Now,loosen the M/C cover, so that you can lift it up a hair and peek into the reservoir.Bring a flashlite. Ok put the cover down,and get the helper.Next have the helper pump the pedal twice, keeping it down hard. You,standing to the side, lift the cover just high enough to shine the light in there and see the fluid. Next, watch carefully, dont take your eyes off the fluid and signal your helper to take his foot right off the pedal. When he does this you should see a little fountain in there as the fluid returns. At the very least you should see the fluid appear to boil.Ok, do it again.I suspect you wont see the fountain nor the boil.Thats proof the fluid is NOT returning.Now you have to figure out why.
-So at this point, the system is under pressure. I say that because of what you previously have said.Now also there may be parts of the system not under pressure, depending on where the restriction is. Since we previously decided the problem likely is at the J-block, lets go there.
-Gather up all the non-contaminated paint protectors, and move them to between the M/C and the J-block area. WE are going to crack each connection starting with the one closest/at the M/C, one at a time, until we find pressure and bingo! Whatever lies between this last cracked connection and the wheels is the problem, unless its the very first one at the M/C. Then the problem is the M/C itself. Be careful.There could be 200 or more psi in there.It will be desperate to get out.Wear eye protection.
-Theres a lot of info here to absorb on the first go-round. Read it as many times as you need to get it.
-I really want for you to come back and say "Bingo, I found it", so happy hunting.
-This is not a complete Diagnoses yet. There may be other mechanical issues at work. I just started with what to me was the most obvious.If you do find the problem is the valve or one of the valves, it will need to come apart and be inspected, cleaned, repaired or replaced.
 
I just started working on the wagon again and still haven't figured out my brake issue. I took the distribution block off and cleaned it inside and out and its still doing it. Here is a picture of what I have looks like.
 

Attachments

  • IMG_6163.jpg
    24.3 KB · Views: 278
As far as I know, the only "small bore" truck master cylinder is a 15/16" unit for a Dakota application, which definitely won't fit your A-body manual brake push-rod.

Did you install the rubber push-rod retainer?

Is the push-rod FULLY seated in the piston?

If you unbolt the master cylinder and it springs forward, the push-rod is pre-loading the piston.
 
Here's what I would do

Remove the master and bleed it "for certain" off the car. Google bench bleeding, there are plenty of hits

Next, make ABSOlutely certain that the piston can fully return to the rear of the master. This is what AJ was talking about above.....if the piston is held forward, it won't pick up additional fluid

If all that seems OK, and you cannot "get" anywhere, try "pre" bleeding the calipers. "How I did it"

Get something with which you can handle brake fluid. Here, I buy syringes at the local farm store. Take something "safe" like a 2x4 and put in the caliper between the piston and the casting ears. "Blow" the piston out against the 2x4. Now fill the caliper with fluid.

Next, "rig" the caliper to the tubing, and wire it up or whatever you need to do to "hold" it someplace, propped up.

Now, just take a C clamp, and retract the piston, with the line disconnected at some point........such as the distro block, until you get fluid backed up to that point.

Don't consider that bled, necessarily, but it will "give you a head" with the pedal.

Rinse, repeat on the other side.

Now you should be able to "pump bleed" the brakes.

I assume? you know NOT to release the pedal with the bleeder open. You either need two people, so that you can CLOSE the bleeder before allowing the pedal to return up, or else you need hose(s) on the open bleeder(s) to keep air from being ingested.

Blow the piston out extended

attachment.php


Below, "pre" filling the caliper. That's a joke, son. Like "pre" drilling holes on This Old House. You cannot drill a hole before you drill it, therefore you cannot "pre" drill a hole!!!! LOL

attachment.php


After hooking up the hose, I kept it "up" while retracting the piston some.
 
DoctorDiff said:
... If you unbolt the master cylinder and it springs forward, the push-rod is pre-loading the piston.
Click to expand...
Yes. People keep mentioning the push-rod length, but that confuses me. The MC piston should always return to its rear stop. The push-rod length just determines where the pedal will wind up sitting. That assumes the pedal isn't hitting a stop. One such possible stop is the brake lamp switch. It should never bottom out there. That is why I suggested removing the brake switch while solving the problem. You then replace the switch and adjust it so it opens but doesn't interfere with pedal motion. But, the OP said there was play in the push-rod when the pedal was released, so sounds like the pedal isn't hitting a stock, but good to verify for us.
 
You must log in or register to reply here.
-

Back
Top