Brake problem question

[RichardJ]

The "Combination Valve" includes the "Metering Valve", "Pressure Differential Switch" and the "Proportioning Valve". Three valves in one. The "Metering Valve" is sometimes called a "Hold-Off Valve".

>>The prop valve has a hold-off valve in it that engages the rear brakes first for light braking to prevent nose dive, so maybe when the hold-off is surpassed and the fronts engage they grab.<<

It is only semantics, but to put it a different way, the "Hold-off Valve" prevents the front brakes from working until pressure reaches a certain point. The pressure point varies by vehicle, but let's say 50-100 psi.
The rear drum brakes are unaffected by the hold-off valve. The rear brakes would act normally, not knowing if the hold-off was bad or even if there was one present. The proportioning valve doesn't play a role until pressure is upwards of 500 psi. The proportioning valve effects rear brakes only, and only above 500 psi.

MC is leaking - bad MC

Differential switch is ON - number one suspect at this point is bad MC.

Bad nose dive sounds like hold-off valve is working and you have little or no rear brakes - bad MC.

I would caution anyone from taking a combination valve apart in hopes of repairing it. If you damage any seals or valves, there are no replacement parts or kits.

[Mdavis83]

Originally Posted by RichardJ

The "Combination Valve" includes the "Metering Valve", "Pressure Differential Switch" and the "Proportioning Valve". Three valves in one. The "Metering Valve" is sometimes called a "Hold-Off Valve".

>>The prop valve has a hold-off valve in it that engages the rear brakes first for light braking to prevent nose dive, so maybe when the hold-off is surpassed and the fronts engage they grab.<<

It is only semantics, but to put it a different way, the "Hold-off Valve" prevents the front brakes from working until pressure reaches a certain point. The pressure point varies by vehicle, but let's say 50-100 psi.
The rear drum brakes are unaffected by the hold-off valve. The rear brakes would act normally, not knowing if the hold-off was bad or even if there was one present. The proportioning valve doesn't play a role until pressure is upwards of 500 psi. The proportioning valve effects rear brakes only, and only above 500 psi.

MC is leaking - bad MC

Differential switch is ON - number one suspect at this point is bad MC.

Bad nose dive sounds like hold-off valve is working and you have little or no rear brakes - bad MC.

I would caution anyone from taking a combination valve apart in hopes of repairing it. If you damage any seals or valves, there are no replacement parts or kits.

Good info. Thank you

[crakarjax]

Originally Posted by RichardJ

I would caution anyone from taking a combination valve apart in hopes of repairing it. If you damage any seals or valves, there are no replacement parts or kits.

You could use general seals of the correct material, and that's the only part I could see breaking on a teardown with moderate carefulness... but also you can just replace it with a new combo valve if it's busted.

Re: replacing the booster just fyi you can replace the booster without pulling the MC all the way off, just leave the lines connected and there are only two bolts connecting it. But if you do get both, look into a MC/booster combo to be sure you have a matching pair and the linkage between the two should be adjusted correctly for you. Good luck!

[Mdavis83]

Originally Posted by crakarjax

You could use general seals of the correct material, and that's the only part I could see breaking on a teardown with moderate carefulness... but also you can just replace it with a new combo valve if it's busted.

Re: replacing the booster just fyi you can replace the booster without pulling the MC all the way off, just leave the lines connected and there are only two bolts connecting it. But if you do get both, look into a MC/booster combo to be sure you have a matching pair and the linkage between the two should be adjusted correctly for you. Good luck!

I haven't found a mc/booster combo that isn't $300 that is specifically for my truck. Probably just gonna get one from oreillys that way if they don't work I can take them back easily without the hassle of return shipping to an online vendor.

[RichardJ]

What crakarjax is saying, JUST replace the MC. You can easily replace the booster later if needed without opening the brake lines and re-bleeding.
Most posters above agree the booster is probably still good.

[Mike_The_Grad]

I did not replace my proportioning valve, but its probably due for replacement now that i think about it. Im not sure how old it is.

I agree with RichardJ, replacement is easier and faster than a rebuild, especially if you are not familiar with the component. IMO brake system components are not something I would want to start practicing rebuilding on.

The reason my brake booster was bad was because my truck sat in my driveway for about 8 years with no hood and no tarp, I even remember removing the little plastic check valve from the booster as it sat.(don't ask me why, I was young and had no idea what I was doing.)

The symptom of a falling brake pedal at a stoplight are 100% to blame on the MC. That's how I figured mine was bad, in addition to the brake fluid trail down the front of the booster and onto my steering shaft and inner fender...

Looking back at what I just went through, I replaced booster before master cylinder, when I should have done the master cylinder first. I think it was because I was a little intimidated by the MC replacement procedure of bench bleeding and complete bleeding of the brake system. It wasn't bad at all. Just make sure you have enough fresh brake fluid available for the flush of the entire brake system. Which is 1 quart to flush out all the old stuff,but then about another quart to refill it and get all the air out. The stuff coming out of my brake lines was black and smelled rank.

I have a few suggestions that may help make things easier and faster if you plan on doing the job yourself like I did. If you don't already have one, I would pickup a handheld vacuum pump,"MightyVac" is a good one, they have them at AZ or Oreillys. Cost about $40 and comes with the attachments for hooking up to the bleed screws. I was able to do all the bleeding by myself with that thing vs. finding someone else to pump the pedal. Keep the cover on the MC while doing this to prevent more air from entering the system, and remember to not let the master cylinder run dry or else you will have to pull it back off and bench bleed it again.
If it's determined that the proportioning valve is bunk too, then replace the MC and the prop. valve at the same time if you can afford it. Save yourself having to get back under the truck and bleeding the system twice.
The booster is easy to replace without even the risk of getting air into the system. I just rigged something from the hood to the master cylinder so I didn't have to worry about bending the brake lines and let it hang there while I replaced the booster and then bolted everything back up. That was the easiest part to replace, which is probably why I started there.
I wish I had a little more experience with bending tubing to make it look a little more aesthetically pleasing but they don't rub anything and they don't leak. So I'm ok with it. They sell pre-made lengths of brake line tubing with the flare nuts and ends double flared already. They are pretty inexpensive, I would get a good double flaring tool to go over the pre-made flare ends again to help eliminate leaks if you don't already have one.

Also, on the end of the prop. valve that is towards the front of the vehicle there is a pressure differential switch that must be keep depressed while you are bleeding the brakes. It's under a rubber cap. I used a piece of aluminum sheet metal from an old camping stove setup that was part of the ladles handle.(I'm quite resourceful with the junk in my garage.) I bent it into an L shape and cut a U shaped channel long enough for the prop. valves front mounting bolt to pass through. And the leg of the L long enough to reach the button. Loosen the bolt, slide in the piece under the nut until it made contact with the button and snugged the bolt back up. Just make sure whatever you use is rigid enough so that the button's center pin does not extend out. Doing this should prevent the brake warning light from turning on in the gauge cluster.

[Mdavis83]

Thank you for all the info! I guess I will replace the mc first and go from there. Hopefully I won't have to replace any break lines, but if I do I can handle that. I work in a tube shop, although we don't have material that small (3/16 I believe it what those are) I can source the material and do it myself.

[Mdavis83]

Not sure why those pics are upside down??

[El Dorado Jim]

Originally Posted by zosoppp

If the truck starts creeping forward while you are holding the pedal down, I'd think it's the MC leaking fluid back to the reservoir rather than a power booster issue.

Or any other leak from the brake lines/slave cylinders/calipers too.

x2

[El Dorado Jim]

Originally Posted by Mdavis83

Thank you for all the info! I guess I will replace the mc first and go from there. Hopefully I won't have to replace any break lines, but if I do I can handle that. I work in a tube shop, although we don't have material that small (3/16 I believe it what those are) I can source the material and do it myself.

you do know that brake lines require a double flare?

[Mdavis83]

Originally Posted by El Dorado Jim

you do know that brake lines require a double flare?

Yes. We do them occasionally at work. Have a kit for several different sizes.