Master cylinder capacity

Over the weekend I put on a set of 11" brakes that I had lying around. They came off my D30 when I went to disk and they only needed slight modification to fit my rear D44. Drove around today and they do stop but not like I did with 10" drums in back. They pedal is kinda mushy feeling and I've bled the brakes twice now.

I'm thinking that either:

a) My master cylinder (4 year old NAPA reman unit) went bad at the same
time as the brake upgrade. Coincidence

b) A stock dual reservoir MC just doesn't give enough flow for both disks
and 11" drums.

What would happen if I used a 4 wheel disk MC (68 vette) with the drums in back? I have Wilwood residual pressure valves plumbed in so that shouldn't be a problem.

 

Joe I don't know about the vette M/C, but are the brake lines in the right place to use the 77-78 Disk brake M/C from Jeep and mount it under the floor.

 

I've got some 11 brakes off a D30 that I was planning on putting on the rear. What kind of mods were required? Looks bolt-on to me. But I know how that goes. I'm actually experiencing the same problem with my disc conversion. Mushy pedal. I plumbed in 2lb residual to front, 10lb residual to rear with Wilwood proportioning valve. I've bled the system twice, but it still feels like there is air in the system. Did discover a little leak at one brake line connection last night, but was too tired to deal with it then. I'm hoping that's the source of the air.

 

Spongy brake normaly mean air in the lines. When I do any brake work on the lines, wheel cylinders or Master Cylinder I allways super bleed the system with a Quart or so of fluid. Works good for flushing the lines and any dirt or moisture you may have got into the system. Fluid is cheap, I hook a tube to the bleeder fitting and pump away working thru all bleeders closest one first so I make sure I don't push air into the long runs. Mark

 

Side note:

I've always been told to start at the wheel farthest away from the MC? Have I alway done it wrong?

 

nope

Joe, I wonder if the one or more of the pistons in the wheel cylinders are stuck/bound up; might be only pushing one of the shoes for contact
also, short shoe to the front (I know you knew that)

 

Well I guess I'll try bleeding them again tonight.

I've got the short shoe to the front. However I think I put the backing plates on the wrong sides as the adjusting port goes to the front. This makes the adjusting cable route to the front of the drum. Because of this I had to switch the adjusting mechanism to the opposite sides (L to R and R to L). It does work like its supposed to still, pulling the adjuster up upon expansion and then moving the star wheel when the shoes contract.

If re-bleeding the brakes still does not work then I may see what it takes to get a 78-86 MC in below the floor.

Thanks fellas!!

 

thinking out loud ( I do that a lot)
the D30 front brakes would/should have big 1-1/8" diameter wheel cylinders
is this throwing your system out of balance? (is there such a thing)
most rear cylinders would have no larger than 1" diam cylinders.
I'm not an engineer (hey I spelled it wright!)

 

Yup, 1-1/8" wheel cylinders. This is why I'm thinking the stock MC doesn't have enough volume along with the disks.

 

I have heard both ways on bleeding them. The reason I start at the closest is when flushing fluid I push out the closest first as not to contaminate the furthest one. When you get air near the Master Cylinder and start at the far one the air may stay in the line. If you started at the closest one it would go out fast. Think cleaning lines by starting close you remove all the fluid till clean and go to the next and push it out, till you get to the furthest and all the fluid should be replaced now in the lines. This is allway how I have done it. Might be wrong but has worked in the past many times and seemed right by logic? Mark