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A little help here please.

Leper

Leper

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Dallas Baby!!!!
My son swapped his 14FF axle to disc brakes. The install went smooth, everything is on and functional. The brackets came from Kert and the Calipers came from Autozone.
The truck is a 79 Blazer 400/350/203/14/44/4.56

Here is the issue: The pedal is extremely low and the truck doesn't stop well any more. We have gravity bled the system, 2-man bled the system, and bled it with the hanging bottle. Even went so far as to re-bench bleed the MC. I dont know why the fronts dont stop as well anymore, because even without the rears, the fronts do most of the stopping.(no we didn't try to block off the rear line and drive it).
Should I try a different MC?(like a bigger one) The MC on his truck is only a few months old, and worked fine until now. Is there a part number for a heavier duty(bolt on) MC that will work? All that I can think of now, is that because the rear calipers are the same size as the fronts, the master cylinder is not moving enough fluid to make them all work properly. Yes the MC has 2 seperate reservoirs.
Any suggestions?
I will be using a power bleeder on the brakes tomorrow night. I would like to have this fixed for him before he gets home from work tomorrow.
TIA
Leper
 
When I did mine, I upgraded the M/C to a 3/4ton m/c of the same yr as my rig. I am still using the stock 1/2ton prop valve, but I've heard of people also having to upgrade that as well.
 
you could get the mc for an older corvette with four wheel disc

my friend has one and it works great

the stock mastercylinder doesnt hold the correct pressure on the disc
 
If you have the same MC I do ('73 D44/12B) The section of the cylinder that runs the rear is much smaller than the front, and therefore will not move the amount of fluid needed to expand the pistons on rear disc calipers. I would upgrade the MC like Muddytazz suggested. That should solve the problem. If you have bled it that much, you shouldnt need to do it again til you swap out the MC.
 
So, get one for a 79 K20? Should I get the proportioning valve(just in case/while I'm doing the rest)
 
I wouldnt worry about the proportioning valve until after you get the new master cylinder, try it and see how it works out. You may want to get one if you feel that the rear will lock up too easy, but either way you should try it without it.
 
You might want to check into a variable proportioning valve after you swap the M/C. Most of the quality conversion kits recommend them and even if you get a different M/C, you will proabably still have problems with braking balance between front and rear.
 
I had the same problem. I went Hydro Boost and will never go back. Pedal is like and rock and locks em down no problem.
 
What calipers did he use? The Caddy calipers require more work to get right, or the pedal will stay low regardless of how much you bleed the brakes. :(
 
There are a couple different schools of thought on this. Yes, you may need a bigger MC, but you also may need a new prop valve. Swapping to disc brakes can sometimes reveal deficiencies in other parts of your system. Usually, when you swap to disc brakes and your pedal stays low, it's a sign that your MC was on it's way out anyway. BUT...once you fill your MC with fluid and bleed the brakes, the system is full. The MC has more than enough fluid and pressure to activate the rear brakes. Very rarely do you need to swap out the MC because it is not big enough. Usually it is because it was getting weak in the first place.

Here is some food for thought. The factory prop valve is set up for a disc/drum brake application. The factory prop valve keeps 2 psi of pressure on the front brakes, and 10 psi on the rear. So, when you convert the rear to disc, you are putting a lot more pressure to the caliper than it needs and holding there. Basically, you are dragging your brakes. This is why sometimes those who convert to rear disc have rear brakes that lock up easily. They also have MC's that are working properly.

Usually the biggest problem with a rear disc conversion is that you are using a prop valve set up for a completely different application. To get around this, many run a manual adjustable valve. It works, but its a PITA.

There is a company out there that sells a brand new direct replacement prop valve set up for the 73-91 blazers (wonder who that might be??;) ) It is modified for the disc/disc conversion. I HIGHLY recommend it for those that are having spongy pedal issues after doing the disc brake swap. Usually, after you swap in this prop valve, all pedal issues and/or grabby rear brakes dissapear. Oh yeah, it's not near as expensive as you think it might be for a factory type prop valve. It's usually in the $120 range. It may seem expensive for a prop valve, but how much is your time worth? It has gotten to the point that I won't bother with an adjustable added to the rear line. Too much time and hassle. Bolt in a go I say.

PM me if you want more info. We have converted literally hundreds of rigs. There aren't many issues that we haven't seen with this conversion.
 
I thought that I had read that drums require more fluid, or pressure, to actuate when compared to discs. The one conversion I worked on had rear lock-up long before the fronts locked, but this set of threads made me think I had it backwards.

As for the prop valve, I stand corrected. If you have a 'fixed' valve that's designed for this application, that beats the heck out of trying to fine-tune an adjustable valve.
 
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