soft pedal after power disc swap
 

Hi Ive asked this question over in my build thread but getting no response so I figured tech is where to ask. link to my build threadhttp://www.67-72chevytrucks.com/vboa...d.php?t=689194
I recently aquired a 1970 gmc 1/2 ton truck it had manual drum brakes on it I drove it like that for 2 months with no issues but was getting tired of the hard pedal. I had a dana 44 front axle out of a wrecked 1985 jeep J10 truck so I robbed its front spindles and rotors and swapped them on along with new gm 1/2 ton calipers with new 1 ton rubber hoses and ran new hard lines to them. swapped in a power brake booster bracket out of a junk yard along with a brand new booster/master cylinder. got a proportioning valve out of a 74 gm truck with disc/drum at the junk yard. I got everything hooked up gravity bled the system them bled with a person pumping/holding the brake while the other loosened the bleeders.
It stops ok with the new setup and now after Ive let it sit then start it up and take it for a drive the first time the brakes are applied I get softy/spongy pedal it still stops just with longer pedal travel. then the rest of the drive the brakes work as they should with no soft pedal. if I dynamite the brakes the backs lock up but not the fronts they work so so ( just not as good as I expected) this is baffeling to me is the proportioniong valve doing its job or getting stuck with only the back brakes working, its the only thing in the system thats not new. I bench bled the master real well before I installed it. Ive done lots of searching and all Im finding are others who have had simliar problems seemed to get soft pedal more often. Mine does it once during the drive and thats it. In the research Ive done others have adjusted the rear brakes and seemed to have fixed the problem while others have lenghtened the push rod from the pedal to the booster for more pedal travel or just redrilled the hole 1" lower. just lost on where to start? here is a pic of ol' Walter for motivation
http://i1220.photobucket.com/albums/...psjr3pv3rg.jpg

Re: soft pedal after power disc swap
 

Welcome to the board. I've had my share of brake problems so I'll let others with more knowledge give you better answers than I can.

Great pic of ol' Walter in his natural element!

Re: soft pedal after power disc swap
 

Well, I can tell you I did all the proverbial correct things shortly after I switched over to a disc front dana60. But here's what I did not do. I did not swap the master out at first. I just remove the rpv off the front reservoir. Brakes worked great. The master went out(it real old) so at that point I decided to upgrade to proper disc/drum master and prop valves... could not stop the rear from locking up on hard braking. I tried every permutation of prop-valve/dist block and I just got beat down. Went back to original distribution block and got another drum/drum master and pulled the rpv off the front reservoir. Now brakes are good again. Remember you need the correct psi rpv to hold pressure to the rear wheel cylinders.

Re: soft pedal after power disc swap
 

Further, I will be taking a swipe at hydro boost this summer. Since I won't be going with a hydraulic winch on this truck. Going electric because I have an old hickey worm gear arriving tomorrow.... nanner nanner:lol:

Re: soft pedal after power disc swap
 

I think my problem might be the metering valve in the prop valve?? When I bled the brakes I'm it sure if I was suppose to hold the nipple in or out??

Re: soft pedal after power disc swap
 

That's just for the warning light when there's a pressure unbalance in the braking system. 99% sure the prop. valve pin should be held in while bleeding. But if it pops out, the brown wire gets grounded which illuminates the brake warning light in the dash. You can usually re-center the pin (which turns off the light) by stomping very hard on the brake pedal.

Re: soft pedal after power disc swap
 

I guess I should have mentioned that the light does come on occasionally when this happens. I also do not have an rpv didn't know it's needed.

Re: soft pedal after power disc swap
 

Give us a picture of the master and prop valve

Re: soft pedal after power disc swap
 

Here is one before I put a new master cylinder on, it's the exact same setup but with a bigger bore and bigger reservoir for the front brakes.
http://i1220.photobucket.com/albums/...psvar46cqi.jpg

I will snap a shot when I get home from work with the new master on it.

Re: soft pedal after power disc swap
 

I don't know the particulars but on my '72 K20 (with factory front discs), the front reservoir goes to the rear port of the prop. valve and then to the rear drums. Rear reservoir goes to the front prop. valve and to the front discs. Also, factory front discs are fed by 3/16" hard lines, and 1/4" hard lines go to the rear drums. Dunno if any of this affects your conversion though....

Re: soft pedal after power disc swap
 

The way I have it setup with the front reservoir (larger reservoir ) goes to the front brakes (3/16 line) and smaller to back brakes.
It's too cold (10*) and dark outside right now to get a decent picture of the new master. I'll have to grab one in the daylight tomorrow

Re: soft pedal after power disc swap
 

This pic shows your alternator fan belt is out of alignment with the engine drive pulley. You will eat belts and reduce efficiency with it like that.

Re: soft pedal after power disc swap
 

I'll take a look at that.

Re: soft pedal after power disc swap
 

Here is a better pic of the new master along with booster and prop valve.
http://i1220.photobucket.com/albums/...ps56axq86m.jpg

Re: soft pedal after power disc swap
 

What order did you bleed the brakes in? Try starting "backwards"--left front, right front, left rear, right rear. And bleed them many, many times. Sometimes this is what it takes.

Re: soft pedal after power disc swap
 

I will bleed them. I didn't find a way to hold the pin in on the prop valve when I bled them before. some old guy at the junk yard told me that when the pin pops out is when you know you've bled them right :rolleyes:
If that doesn't work my father in law works at the stealership and they've got a nice vacuum pump system that works really well at bleeding the brakes.

Re: soft pedal after power disc swap
 

Sorry for some reason I don't have the ability/option to edit my posts so I will reply again to white elephant. I bled them in the order you described didn't know that way was backwards? Always have started with the one closest to the master then worked my way around. I should have a few hours to mess with it today after I get back from hunting elk.

Re: soft pedal after power disc swap
 

How did you reroute the brake lines to accommodate the dual front steel lines and the prop valve? Also, pop the reservoir cap off and snap us a pic

Re: soft pedal after power disc swap
 

Sorry for the late response was out elk hunting all weekend. I rerouted the lines with new 3/16 steel line exactly in the same place as the existing lines with the exception of the drivers side coming up to the prop valve.
Here is a pic with the cap off.

http://i1220.photobucket.com/albums/...psjuc8w9nn.jpg

Re: soft pedal after power disc swap
 

The "normal" way is to start from the furthest from the master cylinder to the closest--right rear, left rear, right front, left front. That's the way "most" recommend doing it. The last time I did it like I suggested before and it worked well.

Re: soft pedal after power disc swap
 

You are correct. The front reservoir on that MC goes to the front of the prop valve with 2 ports. The front 2 port design just removes the "T" joint at the cross member, other wise it works just the same.

Is that the same drum/drum prop valve? If it were me (and I did this) I would get a disc/drum prop valve from a mid 70's truck to be sure I had what I needed.

When bleeding brakes, don't slam the pedal or push real hard. It is just not necessary. I have never "popped" my prop valve while bleeding, only when a line broke, just like it is supposed to work.

Re: soft pedal after power disc swap
 

No the prop valve I have is one I removed from a 73+ gmc with disc/drum from the junk yard. Im thinking about buying the wilwood adjustable one to keep the backs from locking up and maybe cure my soft pedal issue. its so frustrating when it happens.

Re: soft pedal after power disc swap
 

A previous poster mentioned the rear resevour is for the front brakes and the front resevour is for the rear brakes, this is the way my K20 is, i think the rear may push more volume to the brakes?

Heres a pic of my setup:

http://www.67-72chevytrucks.com/vboa...&postcount=185

Re: soft pedal after power disc swap
 

on that master isn't the big reservoir for the disc's and small for the rear drums?

Re: soft pedal after power disc swap
 

Calipers need a greater volume of fluid to operate than wheel cylinders (think large pistons in calipers vs small pistons in wheel cylinders).

With a disc/drum setup, the larger reservoir is for the front calipers -- the smaller reservoir is for the rear drum wheel cylinders.