Rear disc brake conversion help?

[trouble454]

Just converted to a front one ton dana 60 and rear 14 bolt with disc brakes. The front calipers are replacement units bought locally. The rear calipers are front take offs from an 86 two wheel drive pickup. The master cylinder is stock for the rig. The vehicle is a 1979 K-5 Blazer.
The master cylinder is 1 1/8 bore. I have new hoses on the front for the new calipers and I have a new booster.

My problem is I can not get a good positive pedal. I have pressure bled it, I have two person bled it, and I have even taken each caliper off and held it high and tapped on it to try and get any and all air out. The brake pedal is low and if held can be pushed clear to the floor when the booster is helping. It seems as though all four corners work, I just can not get a good pedal.

I have also tried a 1 5/16 booster for the same year one ton with hydroboost and the pedal is the same. I have also plugged one side of the master cylinder to check the prop valve and it will set the light and close off the low pressure side, plugging the opposite side of the master causes the same. I have plugged the back brakes off at the tee on top of the diff eliminating the rear brakes and the pedal is still the same.

What am I missing? What have I overlooked? Any and all help will be greatly appreciated!!!

[Engineer]

I had the same set up on my '77 K-30 D60/14-bolt.

Problem with mine was it always had a squishy pedal even when I first bought it, and it had the drum brakes on it.

I replaced the master cylinder a couple of years ago, and the new master cylinder has a much better feel about it.

The only difference with my set up is that I have 4x4 calipers, brake rotors, and backing plates. I don't see that the fact you are using 2wd calipers should make any difference.

Did your truck get the squishy feel after the 4 wheel disc swap, or was it always this way?

Since rebult master cylinders are only $20~25 bucks, I would try a new master cylinder. It sounds to me like that is your problem. Since there really isn't anything in a 4 wheel disc system to compress, and you are sure you have all of the air out of the lines, I tend to think it is the thing that makes pressure to begin with.

[1tonson]

I was looking at rear disc on my 83" k-30 dually, and notice most of the kits were for off road only ? what gives ?
 Wouldn't this change over from drum-to-disk be better for towing on the street.

[Engineer]

I think they list them off-road only for liability purposes.

I switched to 4wheel disc because my drums needed rebuilt, and I already had a 3/4-ton D44 laying around that was a donor for my calipers, rotors, and backing plates.

Since my set up is home made, there is no one to tell me off-road only.

I would think 4wheel discs would have all the obvious benefits regardless of how they are sourced. I rationalized in my mind that by using the front rotors, and calipers off of a 3/4-ton that you would have atleast the stopping ability that the OE designer of the 3/4-ton trucks had in mind for it.

[79K20]

You sure there are no fluid leaks? Check the routing down the frame to both axles.

I know you said you changed the master cylinder already, but those symptoms sure sound like a bad MC to me. If there are no fluid leaks then it almost has to be a MC issue. What are the odds that both of the were bad? I would install a re-man MC and go from there.

[fitz]

I was looking at rear disc on my 83" k-30 dually, and notice most of the kits were for off road only ? what gives ?

 I think this is because the cheaper kits eliminate the emergency brake.

[Captkaos]

Most say offroad only because of the reason Fitz listed,  No parking brake.

[1tonson]

Hvae you or anyone seen the new issue of 'street trucks'? is an article about an brake upgarde on a 2007 chevy9big brakes0. But they are showing a neat "SSBC" rear brake caliper with emergency brake lever in the caliper.

 I have been trying to find some real world comparisons, for the dually rear drum-to-disc brake swap. Just cant seem to find anything.

 Oh ye, they also hgave an article about how to paint your truck chrome..ME WANTS...

[Captkaos]

I think I have that one.

[trouble454]

I have finally got back to this project.... replaced the master with one for a 78 corvette with four wheel disc brake. The pedal is still the same low and will fade to the floor .  I bench bled the master first... this makes for the fourth master with same problem. When I plug the front side of the brakes at the master the pedal goes halfway down and gets rock hard...also the prop valve sets the light, however I have very little brake as it is easy to drive thru them.
The hoses and calipers on the front are new... is it possible to have a bad front caliper? Is it possible that a brake line will not leak fluid will allow air into the system?
Thanks for any and all suggestions. This is a snowplow rig and its getting to close to having to use it now.

[joesgarage71]

 you still have the prop valve from a disc/drum set up, Mumm
 Check this out.http://www.pirate4x4.com/tech/billavista/14b_disc_brakes/

[joesgarage71]

 This too for the basic stuff
http://www.pirate4x4.com/tech/billavista/Brakes/

 Are you using a master cylinder for disc/disc brakes?

[VileZambonie]

You should be running a quick take up master cylinder and you need proportioning valves not metering valves so the combination valve should be out.

[Zeked]

I found this quote in the archives..... Dont know if it is (a) true (b) causing your problem... but here 'tis

"... the calipers need to be mounted with the bleeder fitting at 12:00 so all the air can be pushed out when bleeding the brakes.  If it's not, you will end up with a spongy pedal.  "

[beastie_3]

Ive never heard that before. It doesnt make sense to me since the brake lines along the frame rails are higher than the calipers anyways. My street bike has bleeder valves pointing down. It might just be the manufacturer of your calipers. Not sure.