Brakes - questions

[53lowdown]

62 C10 with ~ '70 front discs, stock rear drums. Dual Master from 68 Impala disc/drum, and 9 inch aftermarket booster.

I can't get this thing to stop quickly - can't even lock up the wheels on wet pavement, so a panic stop would be a real panic.

1. The Booster seems to be working - Hold pressure on the brake peddle, then start engine and it pulls the peddle down.
2. Pads look fine - hardly worn
3. I've bled the brakes 3 times - and I'm not seeing any bubbles.
4. It stops straight - no pulling to either side

The peddle seems a little mushy but it doesn't keep going down.
Pumping the brake peddle makes only a minor difference.

I was thinking maybe replace the front hydraulic hoses? But fluid seems to be flowing well when the bleeder is open.

Would a bigger booster solve the problem? Seems like even with no booster I should be able to lock them up if I stand on them hard enough.

Any other ideas?

[AcampoDave]

Here are a few questions:
Is the pedal low? Does the push rod completely disengage from the the system when the pedal is at rest? And, if those rubber lines look old just change them. I've seen too many of them go bad internally while the exterior surface still looked o.k, and yes I run disc front brakes with no booster and can easily stomp the pedal and lock'em up.

[53lowdown]

Pedal comes completely up. there's about 1/2 inch before the rod engages. I'm going to replace the hydralic lines. I've had them go bad before too, but usually just one side, so that's confusing the issue here, because the truck stops straight.

[padresag]

do you have a proportioning valve in the system?
ron

[53lowdown]

Yes, I have a proportioning valve on the line to the rear axle, but currently have it wide open.

[padresag]

Originally Posted by 53lowdown

Yes, I have a proportioning valve on the line to the rear axle, but currently have it wide open.

proportioning valve proportions(regulates) the flow between the front and rears brakes on a gm setup. if you didn't have one at all your back brakes would be locking up first
ron

[53lowdown]

Originally Posted by padresag

proportioning valve proportions(regulates) the flow between the front and rears brakes on a gm setup. if you didn't have one at all your back brakes would be locking up first
ron

Right - I have been adjusting the valve to diagnose this problem. From full shut, to wide open, and steps in-between. I can't get the front discs to effectively stop the truck regardless of how I have the valve set.

[Palf70Step]

What did the proportioning valve come off of? Some folks mustakingly pu on a drum brake distro block instead of a true proportion valve. Where are u getting the vacuum pull from?

[padresag]

Originally Posted by 53lowdown

Right - I have been adjusting the valve to diagnose this problem. From full shut, to wide open, and steps in-between. I can't get the front discs to effectively stop the truck regardless of how I have the valve set.

this is an aftermarket p valve that you have?
If you can still build up your pedal pressure you still have air in the system. to bleed l/rear, r/rear, r/front, l/front.
you are sure that it is disc/drum m/c
why not give us a pic of the installation. the frt and rear lines are not mixed backwards are they(this happens)
frt brake hoses usually hold the frt brakes on after you release the pedal if they are faulty
your rear brakes are adj up.
it is usally better to go with an orig type system complete the way it is intended especially in brakes and steering unless you know what it is all about or someone says put this here and it will work. funny thing is that they have never tried it themselves.
if you have a problem sometimes it is better to back to the basic system
ron

[jayoldschool]

I believe the fix will be a proper disc/drum master cylinder, combined with the correct prop valve. Followed by correct and thorough bleeding.

[The Rocknrod]

Many people miss this check and or adjustment:
http://www.mpbrakes.com/techtalk/how...r-cylinder-gap

[padresag]

http://67-72chevytrucks.com/vboard/s...d.php?t=463321
this was in the faq sction
ron

[53lowdown]

Here's my setup. Booster/master and Prop valve pictures attached. The front and rear lines are separate - so I have a common adjustable proportioning valve on the rear system.

I'm putting on new front hydralic lines, then going to re-bleed.

[siggyfreud]

Is your 9" booster single or dual diaphragm?

[53lowdown]

I mistyped - It is a 7" booster, and I believe it is single stage

[siggyfreud]

I think the booster might be your problem then. Let's work our way back from the brakes. I am writing it this way not because I don't think you understand, but just because this is how I think about the problem since I am not a brake expert.

Given enough pressure, the brakes will lock up, so we know you're simply not getting enough force at either the front or rear. So if we start at the calipers/wheel cylinders as the last point where pressure is applied and think about what would prevent the pressure for making it there:

1. Are the cylinders or calipers leaking? Doesn't seem like it, and if they were you'd still be able to lock up something I'd imagine, even for a second before line pressure bled off.
2. Are the flex lines leaking, or expanding. Probably not unless you feel extra movement in the pedal. And even then, you'd think that unless the leak was major you'd get at least SOME of the brakes to lock up.
3. Is there leakage in the hard lines somewhere?
4. Is the prop valve set right? - Well your situation doesn't change based on that, and it should. If nothing else, you should be able to get the rears locked up by having the valve wide open. But you said that's not happening.
5. Is the master cylinder right for a disc/drum setup?
6. Is the master cylinder undersized and not moving enough fluid?
7. Is the booster adequately providing enough assist?

To me, a 7" single stage seems really small for a disc/drum setup. I'd think that you'd want a 10-11" single, or 9" double perhaps for something with disc. I think 9" single is what drum/drum setups usually want.

So I think either your master is wrong/undersized on bore size, or your booster is just too small to give proper assist for a disc/drum setup.