Bigger Master Cylinder after 9" axle swap?

Ok, so now I have the 9 inches in front and rear in the '99. The brakes are all 1978 f150 , fronts are single piston caliper, rear are just drum. I have run 2.5 qts of brake fluid through the damn thing (to make sure there isn't air in the lines) and still have no pedal when its started, cannot find any leaks, and all fittings are snug. The pedal just goes right to the floor, but will build slight pressure if pumped a couple of times.

When the engine is not running it builds pressure, but if you wait 10 seconds the pressure bleeds off slightly.

The brakes worked fine with D30 and 8.25, so cannot figure out for the life of me what the deal is. I am still running the same brake lines , I only removed the lines at the calipers and the junction block on the rear axle.

Theory #1: The larger pistons on the fronts are requiring more volume than the stock m/c can push to maintain line pressure.

Ok, so that's really my only theory.

Any advice would be very helpful.
Anybody have this problem too?

I am considering a 1999 Ram 2500 or 1999 Durango M/C anyhow, but want to try to resolve the problem prior to the new M/C (if possible).

Thanks!

I run chevy half ton calipers in front and chevy passenger car calipers in back (basically a smaller half ton caliper) with the stock booster/master/prop valve, and have a nice high pedal with a great feel to it.


are the rear drums adjusted properly? while doing the swap did you run out of fluid in the master? might be a good idea to start fresh with the hydraulics and bleed the master separetly before moving back to the wheel cylinders than calipers.

not sayin, you didn't do these. but sometimes in desparation I myself forget about the simple things i could be checking.

sounds to me like the rear drums are way out of adj. they should be adj. up so there is very light contact between the drums and the shoes

http://www.pirate4x4.com/forum/showthread.php?t=575345

This might answer what will work for you if your looking to do a master cyl swap.


When you had the lines open to do the swap did the system ever run dry? Its also possible that the master it bad. If you were bleeding it and running the pedal to the floor its possible that the seals in the master got damaged from pushing them further into the bore than they normally went with the system before.

I will check the drums, I think they are adjusted fine but will double check. I did replace one wheel cylinder that "may" have been leaking but the problem is still there. I am pretty sure the m/c was not run dry. I guess I will start at the top of the system and bleed the M/C (after I check the drums) and work my way down the line. I hope it is something very complicated, that way I am not an idiot.

i know this sounds stupid but was a problem for me in a hasty job... i put the calipers on the wrong sides, that put the bleeder valves on top so they made an air pocket.... just one of those easy things to look over...

rockfrog said:

I will check the drums, I think they are adjusted fine but will double check. I did replace one wheel cylinder that "may" have been leaking but the problem is still there. I am pretty sure the m/c was not run dry. I guess I will start at the top of the system and bleed the M/C (after I check the drums) and work my way down the line. I hope it is something very complicated, that way I am not an idiot.

Click to expand...

Did I misunderstand your post?

Do the MC, and even the combination (proportioning) valve, then it is RR/LR/RF/LF.

joe_peters said:

Did I misunderstand your post?

Do the MC, and even the combination (proportioning) valve, then it is RR/LR/RF/LF.

Click to expand...

What I was meaning is I will bleed the M/C then bleed the rest of the system in the normal way you bleed brakes.

aintright said:

i know this sounds stupid but was a problem for me in a hasty job... i put the calipers on the wrong sides, that put the bleeder valves on top so they made an air pocket.... just one of those easy things to look over...

Click to expand...

Arent the bleeders supposed to be on top so they BLEED the air out of the top of the caliper.

yes, you are right, sorry being a ratard again, when they were upside down the bleeder was on the bottom leaving an air bubble at the top....

shit, see what i mean about the simple stuff

Ok, so the drums were out of adjustment, I did 1 full turn on the dial and what a difference. So, now that it is actually getting good pressure to all four, I found a leak at the left front hard line into the soft line. I think I got a little over zealous tightening it when I was pissed at the whole thing and damage the line. Does any body know what size that line is? I would try to reflare it but can't find my flare kit, would hit the bone yard to get the line but the high tomorrrow is suppose to be 10 degrees. So off to Napa I will go.

Thanks for the help, I really do wish it was something more complex that way I wouldn't feel like a moron. I can honestly say it has been 5+ years since I tinkered with anything that had drums on the rear.