dual MC, disc upgrade problem

I have got the disk conversion and dual MC conversion on my 68 w/ dana 27 essentially complete. However I have bled the brakes, tightened fittings,and bled again, and still have a very spongy pedal. It goes to the floor and I dont see any fluid coming out anywhere. Is my MC pushrod adjusted wrong or something? I inserted it, marked it, and cut it down, then added new threads. I adjusted it so that when it was inserted into the MC, the ball end just slipped over when the pedal was all the way up. I installed the Wilwood 2lb and 10 lb inline RPVs but had to install my own fittings, they dont seem to be leaking though. Am I missing something here?

Josh

 

The first question that usually gets asked for this is "did you bench bleed the new master?" If not it will take longer than normal to bleed the system by the usual boy and man method.

I had a problem once on my rig that involved the position of the bleeder at the brake caliper - it wasn't at the high point so would not bleed air. I ended up taking that one caliper off and rotating it so the bleeder was at the top to get the air out. You shouldn't have to do this but I had two left calipers so that was my problem. In your case maybe you have the calipers reversed?

If your MC pushrod is wrong I guess it could be too short? So it doesn't push all the fluid (and air) out? I'm not sure how to tell - maybe take it apart and see how far the rod could go in? or how far it needs to go in for a full stroke?

 

No, the bleeders are definately facing up. I double checked to make sure the L was on the left and the R was on the right. I will double check pushrod length. I never understood the bench bleed of the MC. You get the air out on the bench then un- hook the lines to put the jeep lines on - letting air in. I think this is what happened to me. I will bleed it and bleed it some more tommorrow.

Josh

 

Bench bleeding is very critical. Did the M/C come with line plugs? I bench bled mine and used the supplied plugs to install it.
How do you have the brake lines running? I did this conversion and had the same issue. Bench bled M/C, bled and re-bled the brakes but the pedal went all the way to the floor. I finally found a thread here http://www.earlycj5.com/forums/showthread.php?t=26819&page=5 (page 5, post #45) about running the front brake line to the rear or first resevoir and rear brake line to front or second resevoir. Corrected the problem immediately and they work great.

 

Josh,
The idea of bench bleeding the master cylinder is to get the air out of the cylinder. When you install it if you are using plugs you might get some air in there but mostly at the outlet ports. It really can make a huge difference. You can do the same thing on the vehicle, just remove the lines at the master cylinder ports, Install fittings in the ports with hoses going into the reservoir, then bleed all the air out, then re-install the lines. It's usually easier to do on the bench with an under floor master cylinder but still can be done on the vehicle.... Nickmil

 

I did bench bleed. The MC sat for a little while before I got it up in there and connected.I didnt have any plugs to put in. I bled all 4 wheels again this morning. I can drive and stop but sometimes I have to double pump the pedal to do so. Yes I installed the RPVs and in the right direction. I tried slightly opening the fittings one at a time at the MC while pressure was applied to let any air out all I got was fluid. ?????
Josh

 

Are the rear brakes drum and are they adjusted up correctly? If out of adjustment it can cause the symptoms you describe......
Does the pedal sink to floor steadily or is it firm when this happens...

 

Oh, by the way- When they say mount the rear brake line system to the front chamber and mount the front brake line system to the rear chamber, they mean front as it is mounted on the jeep (front of the vehicle) correct??
Josh

 

What are the chances my new MC is not big enough for the larger caliper pistons up front? The MC is a 67-71 dual chamber frame mount MC. It is the same as the MC in Steve Lane's writeup so I figured it would work because it worked for him.

Josh

 

Yes correct.
My understanding is you want the first resevoir the rod pushes to go to the front brakes. If you look at modern cars that is the way most are set up. I had exactly the same problems you describe. I originally had the brake lines reversed. When I set them up as stated in the thread I had full pedal and could actually lock up the front brakes.

 

It's possible, but I'm running the stock under floor master cylinder with 4 wheel disc brakes using the large GM calipers and it works great so yours should work fine. I wonder if you have a mc that's bypassing internally....
Nickmil

 

I ended up buying a new M/C from a local parts house. After getting into the conversion I found out the M/C was leaking. It looked exactly the same as the one I took off. Don't have a PN though. I might have the box in the garage....I'll go check.

 

Found it...PN: Raybestos MC 36365

 

I think next I'm going to try the re-bleed the MC on the frame procedure, with disconnecting the lines and running the tubes up to the reservoirs. It could be that even though Im not getting air at the wheels, air is trapped up there.
Josh

 

Is your MC new? Had a simillar problem using rebuilt MC. Finnaly bought a new one and followed the directions for bench bleeding. Its been working great for a year now. Get a new MC.

 

Just wanted to add my two cents worth.
On duel reserivor master cylinders. The largest of the two reserivors is always the one used for the front brakes. As the pistons in the calipers move out to compensate for the pad wear they require more fluid.

 

OK. The advice I am getting from various people is that the piston in this MC just isnt big enough to push all the fluid needed for the huge pistons in the calipers. The dual chamber (which I am using) is for a 67-71 is for 4 drum brakes which have smaller piston wheel cylinders up front. That is why I have to pump the brakes to stop. I dunno, this is the advice I am getting.

?? Josh

 

Have you talked to Herm (Hermtheoverdriveguy)?
I looked over my original M/C and the replacement and could not see noticeable differences but.....there may be some. How do you have the brake lines running? Did you bench bleed the M/C until you could hardly push it by hand? I think mine took the better part of an hour to bench bleed. Believe me if you read my thread I was at least as frustrated as you when I did the swap....but well worth it now. Just my $.02.

 

I'm using my stock 71 dual MC for my front disk brakes and have no problems stopping at all. The factory dual MC has a 7/8" and 1" piston, it is one of the very few MC that actually has a stepped piston. Bench bleed it and make sure your large reservoir is feeding the disk brakes. I have a stock dana 30 from a 77 cj5 with factory disks. When I had the small reservoir on the MC hooked to the disks it would not stop at all.

Does your dual MC have a bolt on lid, that is factory, if it has a wire bail to hold the lid on its not a factory style, at least not for a 69-71.