Technical Weak brakes 61 Chevy tk Gm Discs

I have a 61 chevy tk project that I converted to front discs with an 87 front crossmember, also installed the later drum brake rear out of the same 87 parts truck.
I used a new mid 70's impala booster and 1-1/8" bore master cylinder, both reservoirs are the same size.
New calipers, hoses, wheel cylinders, shoes, steel lines, drums are within spec rear shoes adjusted up.
The proportion valve is out of the 87 parts truck.
I got it running and road tested it and the brakes feel weak, the fronts will barely lock up on gravel and the rears do not on gravel but do lock up in the air
I have bled the brakes a couple times and am getting good fluid out all the bleeders.
I adjusted the rod from the booster into the rear of the master cyl to take up the slack, which did bring the pedal up, but no improvement in stopping performance.
This 87 had one of those plastic aluminum master cylinders and from what I've read they have a 26mm/40mm bore (1-1/8 - 1.57") I'll admit I don't know much about these.
Are the proportion valves different on these later square trucks?
Should I get rid of the factory valve and put in an adjustable one?
In my mind this all standard 71-87 GM stuff and should work
Any help or ideas would be appreciated.
Thanks,Bud

 

which pads did you use? i remember fighting to get a good pedal on those trucks when the semi metallic, lifetime and ceramic pads came out. we would switch to a "cheap" organic pad and they would stop better. i am not saying that is your problem.

 

It could be that your pedal ratio isn't the same as the donor truck was. Pedal pad to pivot point.

 

Independent of your current issue just a reminder to leave a slight amount of free movement in that internal master/booster rod.
I did a very similar disc swap on my 66 Suburban, during the first test drive after a quick blast down the freeway I noticed the truck slowing down coming up the ramp to the stop.
I made the rod from a longer one and missed the length slightly, if I recall, I turned about 1/8" off the rod and machined a new radius on it, that did the trick.
Hope you fix your problem.
In case you didn't know it there is a 60-66 Chevy truck thread here on the HAMB, may be worth posting there also.
https://www.jalopyjournal.com/forum...10-chevy-truck-picture-thread.108866/page-114

 

Sounds like you need the truck master cyl.
The factory didn't guess at those sizes.

 

I agree with Unkl Ian on the truck master cylinder. My '63 was "late-modeled" using the complete '79 Square Body front and rear suspension including the column, master cylinder and brake booster, steers and stops great.

 

Could it be that you are not getting enough vacuum to power the new setup?

 

A weak pedal could also be caused by the wrong pedal ratio. If I remember right you want around 5-6 :1 with power brakes and 7:1 with manuals, MC bore size comes into play here aswell. My guess is your mc is too small and can´t feed enough fluid to get a good pedal and you hit the floor or ot bottoms out before the brakes lock up. Had this issue on my 34 when I got it. it did not brake at all. It had a brandnew 7/8 " MC, now I run a 1 1/8 and it is perfect!

 

I bet you have one of 2 possible things wrong. Pedal ratio and master cylinder bore. You probably are not getting enough fluid pressure to clamp the pads to the rotors, and shoes to drum. Go to a 1" bore master cylinder and your fluid pressure will go up. Or move the hole in the pedal where the rod to the master cylinder goes, closer to the pivot point. This will give you more mechanical advantage to push the 1 1/8" piston in the master cylinder.

 

Never thought of the pedal ratio, I'll have to check that
The booster rod has approx. 1/4" distance before applying the cylinder piston
The master cyl bore is 1-1/8" which is the same as a mid 70's c10,
One thing I did notice is this cylinder has the same size reservoirs and the images for the truck ones show a large and small reservoir
I would like to avoid a plastic aluminum master cyl
Thanks for the help

 

First, normal clearance between the booster output rod and master cylinder primary piston is closer to .040". 1/4" is way too much. This is measured with full vacuum applied to the booster.
Second, fluid reservoirs may appear to be the same size, but most dual masters are not 50/50 in operation, so having the front brakes running off a rear brake circuit can cause noticeable front brake issues.

 

Multiply 1/4 by the pedal ratio, and you get..... ?

Paint the plastic/aluminum piece.
Or find one the right size.

 

Thanks for the replies and help, it's appreciated Bud