Weak brakes 61 c10 disc upgrade

[buds56]

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. Capt.Fab bracket.
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. pedal is stock 61.
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

[PGSigns]

Couple of things. First make sure the master cylinder you used in for sure a disc brake master. Dis you move the mount hole down an inch on the pedal? I have done this on all my swaps to get a bit more travel and it helps if manually bleeding. the system. Is the pedal firm or a bit soft? Have you verified you have all the lines going to the right places and the proportioning valve is centered when bleeding the brakes?
Jimmy

[buds56]

The booster was ordered for a 74-76 full size impala, caprice and the master came with it. they were all discbrake cars.

I never thought of the pedal ratio as the booster rod to the pedal was level, I just used the hole that was there.
I'll have to check that.

The lines on the prop valve are routed the same as they were on the 87

Never checked to see if it was centered, but I am getting good fluid flow from each bleeder.

What puzzles me is this impala cyl is 1-1/8" bore with same size reservoirs and a similar chev tk from the mid 70's is also 1-1/8' bore, but the tk master cyl has a small and large reservoir.
I'm thinking of the possibility of receiving the wrong master cylinder ( I bought as a set)
Thanks, Bud

[PGSigns]

Have you got full pedal?
Jimmy

[buds56]

No, it's about halfway down,

The booster feels like its working as it doesn't take a lot of pressure to slow down, but the harder I push slows it down faster but it should lock them up.

[buds56]

Also doesn't feel real solid in the pedal

[theastronaut]

What type of pads did you use? Ceramic pads won't bite at hard and need more pedal pressure to make it stop.

Did you get new rotors, or turn the old ones? New pads on old rotors can take awhile to wear in, especially if there are small ridges on the rotor surface. I've done it with a track car and it works, but that's putting a lot of heat and wear into the brakes anyway so they wear in quickly. It takes longer with street use, so it's best to turn the rotors.

Did you bed-in the new pads? I like to get up to 70ish and brake fairly hard down to about 20 mph, then do that a few times to heat the pads/rotors up enough to transfer pad material onto the surface of the rotor- that improves the coefficient of friction between the pads and rotor. Keep rolling the whole time you do this until the brakes are cooled off completely or else you'll have pad build up where the rotor stopped on the hot pads.

[chevy_mike]

You'll need to drill a hole 1" lower than the factory hole in the brake pedal. You pedal ratio is wrong and the symptoms you are describing sound exactly like that issue. I have had it myself and my buddy too. Once the correct ratio is setup, works perfect.

[buds56]

Thanks for the help and replies, I appreciate it

[jayoldschool]

71-76 full size B body master: front port to REAR brakes IIRC (unlike 60s master). I can check my 71 out in the garage, and I have the FSM as well.

[64shortbox]

A couple of things I can think of. As mentioned earlier You need to make sure the Proportioning Valve or Combination Valve as GM calles it is centered. Since you used a factory Proportioning Valve out of an 87 - remove the plastic switch with the brown wire off of it & check that the valve is centered - the valve will have a little V grove in it. If it's not centered reach in with a small straight screwdriver and center the valve. Now you need to hold the valve centered while you bleed the brakes - there's a special little tool for this - cheap enough - something like $5.00 - I got mine n eBay. Once the valve is centered just screw the tool in where the brake switch was - bleed the brakes again and you should be good.

Just for a little background on my end I used CaptanFab's Booster Bracket, Booster from an 85 C-10, Master Cylinder, Proportioning Valve, and ALL brakes lines from a 73 C-10 & 6 lug Rotors from CPP but stock 64 C-10 rear end & my brakes work as they should great. Oh, I even went so far as mounting the 73 proportioning Valve in the factory 73 Location.

The first 2 Pics are of a cutaway of a GM Proportioning Valve, the second is a Pic of the tool i used to hold the valve centered & the rest are pics of the 73 C-10 brake lines & Proportioning Valve I used all mounted in the factory 73 locations. - Hope this info and pics are of some help.

[buds56]

Thanks for the pics, pretty interesting, Bud

[PGSigns]

So where are you in Virginia?
Jimmy

[buds56]

I live in Manassas

[jayoldschool]

So, how is it plumbed?

[buds56]

Finally got a chance to get back to the truck.

It's plumbed front chamber to the front and rear to rear

I pulled the pedal and drilled a hole about an inch lower and it took care of the problem.

Thanks for all the help, Bud

[chevy_mike]

Originally Posted by buds56

Finally got a chance to get back to the truck.

It's plumbed front chamber to the front and rear to rear

I pulled the pedal and drilled a hole about an inch lower and it took care of the problem.

Thanks for all the help, Bud

Great news.