The Stovebolt Site Forums The Shop Area Driveline Dual Master Cylinder Conversion Problem

First things first. I have a 1949 GMC 3/4 ton with all new brakes lines, wheel cylinders and brakes are adjusted up properly. It has the OEM 1/4 inch steel lines and Huck Brakes.

Recently the old single chamber master cylinder started leaking so decided it was time to move to a dual chamber MC. Ordered the conversion mounting kit and picked up a remanufactured Fenco M-1505 master cylinder (fits 67 Nova) and shows as master cylinder for all drum brakes. Put extra new lines to existing brake lines using the proper couplers. Eliminated the junction blocks that were on the OEM brake lines. The first Fenco M1505 master cylinder leaked out of the front, so bench bled and installed the second one. The problem is that bleeding has been a real pain. It seems that the master cylinder does not put out much volume and this has really made it hard to get any braking capacity. My friend helping me bleed the brakes commented on how long it was taking also. Surely I do not have another bad master cylinder, but looking for ideas. The Fenco M1505 has a 1 inch bore so do not think that volume should be an issue.

Wondering if I should go with a different master cylinder. If so, part numbers would be great.

Any ideas would be appreciated.

Is the plunger going deep enough into the piston? If the master cylinder has a deep recess where the plunger pushes the piston maybe there is too much of a gap and therefore too much travel before the mc is actually worked.

The plunger seems to be going in pretty far. The push rod is against the front of the piston when the brake pedal is all the way up and there does not seem to be a lot of pedal play. There is quite a bit of adjustment left on the push rod, so I may need to lengthen it some more. The pedal also seems to have pressure fairly quickly, but certainly more after the pedal is over half way down. I will double check the push rod length tomorrow to make sure.

I checked the plunger going into the master cylinder and tightened it up about 5/16 inch so there was no pedal play. Still can not get it to pump up. Finally opened a bleeder valve and stuck a hose on it, then pumped master cylinder twice. Not much fluid coming out and had bled that cylinder before. I think I have another bad master cylinder. The bore on this master cylinder is 1 inch and 3/4 ton pickups of that year had a master cylinder with a 1.125 inch bore. Looks like about all else is the same, so I may just go with the bigger bore MC. Is there another test for the master cylinder that I could try? Never had this much trouble with brakes on any vehicle ever before and have been turning wrenches for over 55 years.

Any thought on going with a bigger bore Master Cylinder? Cardon and Fenco both show 10-1331 part numbers.

Two things...Did you eliminate 2 junction blocks and run longer lines? Don't know how you did that.


About 8 years ago I did the dual MC on my '54 3100 and had similar problem. Turns out the plunger rod from mounting kit was to long. Company sent wrong length by 3/4". After confirming with CPP, cut it down & rounded end on bench grinder. Problem solved. I did keep original junction blocks and New 3/16" & 1/4" lines.

Should be some free play in pedal ??? I did not have the Hucks so I may be all wet.

Did you do a search on brake problems ? I find answers all time.

Good Luck!

I ran one new line to the junction block area and just replaced the junction block with a coupler for the front brake. I ran a new steel line and hooked to the rear brake line using another coupler. Ended up just replacing the front junction block with couplers for each line. I plan on keeping the original junction blocks since they are expensive to replace.

Interesting thought about the plunger rod. I am using the rods supplied with the conversion adapter kit, but it is the same length as the OEM rod with the single chamber MC. Not getting much flow no matter where the pedal cycle falls. In fact, there was not much flow when I was bench bleeding the unit. Ordered a MC for a 3/4 ton truck that should bolt right up and will pick it up Sunday. The local Autozone has been really good about the problems and said they would take the defective one back with no problems. Never had issues with reman products before, so wondering what else is going on. Have double checked all lines for kinks or restrictions, but all are good. Used a tubing bender to keep my bends smooth.

I did some searches for info on dual chamber master cylinders, but will go back and try again using brake problems in the search function.

Thanks for the suggestions! Will let you know when I resolve the problem.

My kit had a pushrod that was 1 1/2 " too long. Same thing- cut it down and rounded end. Worked fine.

Got my adapter kit from Jim Carter Truck Parts. Where did you get yours? Put new Fenco 10-1331 MC(designed for a 3/4 ton pickup) in this afternoon, but will have to wait on neighbor for help bleeding. Push rod was only about 4 inches long with over an inch of threads. When run all the way in the coupler, the whole thing is about 4 inches long.

When I bench bled this master cylinder, it was moving fluid pretty well, so think this one will work once brakes are bled. Will keep you posted.

Still curious where you got your adapter kit.

Got my 1st one about 8 years ago for the '54 3100 direct from ECI in Vernon, CT. Saw their ad in Classic Trucks Magazine.

2nd one for my '54 3800 I bought from Jim Carter. Both looked to be same brand and work good...just had to trim the 1st.

Almost 100% sure ECI...not CPP

Finally got around to installing the latest Master Cylinder. I selected a Fenco 10-1331 or M1331 which is specifically for a 3/4 ton pickup. The first two reman master cyclinders did not work correctly. One leaked fluid from the front and the second would not move much fluid even after bench bleeding. Never had any problem with reman stuff before, so was wondering if something else was the problem. The Fenco 10-1331 is working well so far. It still seems to have a little air in the lines, but the brakes are pretty good. Will take it out for a little drive during the next warm day to really check them out.

I think moving to a master cylinder designed for the 3/4 ton truck (1970 Chevy 3/4 ton) I have has helped. The bore on this master cylinder is 1.125 instead of 1.0 inches. The extra bore does definitely move more fluid which is needed for the bigger wheel cylinders on this truck. The rod length from the adapter kit worked well. If it were shortened, it would not activate both chambers of the master cylinder. I have about 1/2 inch pedal play as it is set up now, so think it is about right.

Thanks for all of the suggestions and help!!