New Master Cylinder install issues

[bob_o]

I searched for answers already with no luck, but here's what's up. I installed a new MC and front soft lines on my non-power assist '75 GMC half ton (disc up front, drums in the back, stock system). After that was done, I proceeded to bleed the system using a small hand vacuum pump. There doesn't seem to be any fluid (or air) moving out of the bleeder valves at the wheels. I seem to remember reading somewhere that there is a release button (or something to that effect) on the proportioning valve? There is a little rubber boot covering something on the driver side of the PV, when when pressed, there doesn't feel to be anything underneath it.

Please advise, I'm running out of daylight and I have to drive this thing to work in the morning! Any help would be greatly appreciated.

[geezer#99]

The button you want to press and hold in while you bleed is on the front of the prop valve.

[bob_o]

Here's the PV, mounted just aft of the radiator down on the frame. The arrow marks the location of what i thought was the button, which I tried holding down while using the little hand pump. No luck.

[geezer#99]

Reset should be behind that cap.
Did you bench bleed the master?

[bob_o]

That's what I was hoping, but there doesn't seem to be any kind of resistance behind it (like perhaps the button is damaged?).

Yep. Bleed each reservoir. Though admittedly, I had never done a bench bleed before, so perhaps I didn't do it properly there's still some air in the Master?

[Sabaka454]

Bench bleed it

[geezer#99]

If you got a bleed kit you can bleed the master on the truck. And that button needs to be kept depressed. Sometimes you need to loosen the brake light warning switch to get the button to move.
Did you use a bleed kit to bench bleed with?

[bob_o]

Didn't use my bleed kit to bench bleed the Master. Went by what the Haynes manual said and capped the outlets, pumped the piston by hand with a big Philips screwdriver, releasing pressure on each cap until I had a steady stream of fluid coming from each outlet.

So I'm guessing I can put some barbed fittings into one outlet at a time (keeping the other capped) and bleed the Master with my kit?

[geezer#99]

Do both sides at the same time. Make sure the tubes stay in liquid so you don't draw any air in.
Your kit should come with two tubes and fittings plus a clip to hold the tubes inside.
I've used one like this.
http://www.autozone.com/miscellaneou...it/226332_0_0/

Go slow, keep the tubes submersed.

[bob_o]

Awesome, thanks for all the help Geezer. I'll give this a try when I get home!

[SCOTI]

FYI.... Not all new master cyl's come w/the old style bleeder kit (fittings & short hoses). Last 2 I've done had solid plastic plugs.

[Just call me Sean]

They don't give you the bleeder kit with the hoses anymore, just caps. You bench bleed until you can't push the piston in anymore.

[bob_o]

Yep, the new master only had solid plastic plugs. Figured I would just make some tubes with fittings and tubing I can grab from work.

[Just call me Sean]

You don't have to do that. Just plug and pump. The bleeder kit should have instructions with it, but that's it. Plug and pump.

[Just call me Sean]

As far as the proportioning valve goes, there is nothing to push/pull. I see a lot of confusion here about that. If your truck has a dual reservoir master cylinder it doesn't use that type, front and rear have separate reservoirs so a failure in one will not drain both. Older vehicles with the single reservoir master cylinder have the push/pull type. A failure in the front or rear would drain the only reservoir, so instead when a leak is noticed the valve shuts off the leaking side so it doesn't drain all the brakes. It's the older more primitive system.

[SCOTI]

Originally Posted by Just call me Sean

As far as the proportioning valve goes, there is nothing to push/pull. I see a lot of confusion here about that. If your truck has a dual reservoir master cylinder it doesn't use that type, front and rear have separate reservoirs so a failure in one will not drain both. Older vehicles with the single reservoir master cylinder have the push/pull type. A failure in the front or rear would drain the only reservoir, so instead when a leak is noticed the valve shuts off the leaking side so it doesn't drain all the brakes. It's the older more primitive system.

Not to dismiss what you're saying but I read the valve can stick one way when it senses sudden pressure spikes. I've had more than one stick after e-stops w/a load behind me. Each time I had to crack open the lines to get it to 'un-stick' (when it stuck, it resulted in no rear brakes).

Not sure if bleeding could somehow replicate the scenario.

[Just call me Sean]

Originally Posted by SCOTI

Not to dismiss what you're saying but I read the valve can stick one way when it senses sudden pressure spikes. I've had more than one stick after e-stops w/a load behind me. Each time I had to crack open the lines to get it to 'un-stick' (when it stuck, it resulted in no rear brakes).

Not sure if bleeding could somehow replicate the scenario.

The old style, sure. Bleeding replicates a leak which shuts off that circuit.

[SCOTI]

Originally Posted by Just call me Sean

The old style, sure. Bleeding replicates a leak which shuts off that circuit.

The 'failure' (sticking) occured on 73-87 style prop valves.

My first experience w/the issue was on my 68 (that used a 73-80 disc brake set-up); the 2nd time was on my 74 (original parts).

The 68's we were able to reset (for lack of a better descript) & get flow going again to the rears. We never could get the 74's unit to allow bleeding the rears so I bought another prop valve from Inline Tube.

[Just call me Sean]

Yes, stuff fails in weird ways. The difference is yours was failing while the older style would just be doing the job it was designed to do. No reset on yours but bleeding unstuck it.

[SCOTI]

Originally Posted by Just call me Sean

Yes, stuff fails in weird ways. The difference is yours was failing while the older style would just be doing the job it was designed to do. No reset on yours but bleeding unstuck it.

That is my conclusion (not knowing what else it could have been). I would really like to understand the fluid dynamics of the internals to validate though.

[Just call me Sean]

Can't help with that. I don't know how they work entirely but I do know the two are different.

[bob_o]

Bench bled the MC while it was still bolted to the firewall using the little plastic plugs. Problem solved. Much thanks everybody for the help