What's this dohicky

[Sgt Gus]

So, the rear brake line feed this dohicky that is attached to the rear diff. It then goes around to a t-fitting and feeds both rear drum cylinders.
Reason I'm asking is I'm pushing denatured alcohol through to clean lines before attaching to new cylinders. We good men?
Truthfully, mechanically it doesn't look like it does a damn thing and is just there to f with my head!

 

[Bextreme04]

So, the rear brake line feed this dohicky that is attached to the rear diff. It then goes around to a t-fitting and feeds both rear drum cylinders.
Reason I'm asking is I'm pushing denatured alcohol through to clean lines before attaching to new cylinders. We okay men?
You need a better pic?

Its a load proportioning valve. It limits the amount of braking action on the rear brakes when unloaded so that they don't lock up, while still allowing max braking when loaded heavy in the bed. If you are trying to flush the system, I would disconnect the lever from the diff and put the arm as high up as possible to allow max flow to the rear brake cylinders.

 

[Sgt Gus]

Its a load proportioning valve. It limits the amount of braking action on the rear brakes when unloaded so that they don't lock up, while still allowing max braking when loaded heavy in the bed. If you are trying to flush the system, I would disconnect the lever from the diff and put the arm as high up as possible to allow max flow to the rear brake cylinders.

Ah, ok. Lever is disconnected from diff since we're changing oil. So lifting it up obviously opens the valve...

 

[Sgt Gus]

Sweet, make total sense now. Thanks Brother! @Bextreme04

 

[Sgt Gus]

I didn't think about running denatured alcohol through that weight proportioning valve though...I did push air through to clean out. Hopefully any o-ring etc are ok.
We also figured out master cylinder shot it had been sitting empty for the rears for so long. Picking up new delco booster and master today. Master muddy in the bottom of front. Two fills in back reservoir disappeared. Likely leaking into booster, but didn't find anything by putting a feeler into vacuum port. Interesting. Replace all old = safe plan.

 

[Bextreme04]

I didn't think about running denatured alcohol through that weight proportioning valve though...I did push air through to clean out. Hopefully any o-ring etc are ok.
We also figured out master cylinder shot it had been sitting empty for the rears for so long. Picking up new delco booster and master today. Master muddy in the bottom of front. Two fills in back reservoir disappeared. Likely leaking into booster, but didn't find anything by putting a feeler into vacuum port. Interesting. Replace all old = safe plan.

There isn't an opening into the booster unless the diaphragm is busted. If it did leak out the back of the master it will be stuck back there until you remove the master and then it will all pour out. Make sure you spray it all clean with brake cleaner before you bolt the new one back on. If everything has been sitting for an extended amount of time, it is actually more likely that it all ran out one of the wheel cylinders. Just go one part at a time and you'll get it all sorted out.

 

[spedigo]

There isn't an opening into the booster unless the diaphragm is busted. If it did leak out the back of the master it will be stuck back there until you remove the master and then it will all pour out. Make sure you spray it all clean with brake cleaner before you bolt the new one back on. If everything has been sitting for an extended amount of time, it is actually more likely that it all ran out one of the wheel cylinders. Just go one part at a time and you'll get it all sorted out.

i’ve got a leak between the master cylinder and the booster. It’s running down the bottom of the booster.

Is that most likely just an O-ring or seal on one of the Pistons?

no idea how old that master cylinder is based on its condition when I bought the truck I doubt it’s more than a couple of years oldand

 

[83Stepper]

i’ve got a leak between the master cylinder and the booster. It’s running down the bottom of the booster.

Is that most likely just the diaphragm? Can I just replace that and probably be good? Or would it be any of the other seals with the Pistons?

no idea how old that master cylinder is based on its condition when I bought the truck I doubt it’s more than a couple of years oldand

The piston on the master is leaking. Could be any of the piston o-rings weeping by. Or could just simply be something keeping it open. You can always try taking it apart and checking the bore and o-rings for any crud inside of it and putting it back together, or just save time and get a new master, bench bleed it and install.

 

[Bextreme04]

i’ve got a leak between the master cylinder and the booster. It’s running down the bottom of the booster.

Is that most likely just an O-ring or seal on one of the Pistons?

no idea how old that master cylinder is based on its condition when I bought the truck I doubt it’s more than a couple of years oldand

Agree with what @83Stepper said. I would personally just replace the master. The booster diaphram is also now contaminated, and if it has been leaking for any amount of time it will likely fail soon as well. I'd consider just replacing both at the same time.

 

[Grit dog]

So, the rear brake line feed this dohicky that is attached to the rear diff. It then goes around to a t-fitting and feeds both rear drum cylinders.
Reason I'm asking is I'm pushing denatured alcohol through to clean lines before attaching to new cylinders. We good men?
Truthfully, mechanically it doesn't look like it does a damn thing and is just there to f with my head!

Old post I know, but the difference in the LPSV restricting fluid for unloaded driving vs loaded (rotated cam to full fluid) is SIGNIFICANT.
Mine was disconnected from the axle when purchased.
I noticed sometimes the rear brakes were normal and sometimes they grabbed hard. I could kind of repeat the difference by rotating the valve.
I can see where full fluid is too much rear braking for an empty truck on hard pizza cutters. But with big wide tires it’s really nice. Yes they will lock up a bit quicker than expected but damn do they do some work at full fluid!
One of these days I’m gonna bypass the LPSV because big low pressure 35s have enough grip to make the brakes feel like a new diesel pickup when those big drums are giving 100%.

 

[Sgt Gus]

@Grit dog , since rebuilding rear brakes, new proportioning valve, master cylinder, I was considering this valve could be causing a problem. Some suggest e-brake cables, but lifted I can't get that to stick, or using brake pedal. On the ground rear would grab, and tapping brake seemed to get to release. I haven't done anything since.
Next week take truck to engine builder to replace lifters...

 

[Grit dog]

@Grit dog , since rebuilding rear brakes, new proportioning valve, master cylinder, I was considering this valve could be causing a problem. Some suggest e-brake cables, but lifted I can't get that to stick, or using brake pedal. On the ground rear would grab, and tapping brake seemed to get to release. I haven't done anything since.
Next week take truck to engine builder to replace lifters...

I’m sorry I don’t understand what you’re saying. It was/is a problem?
I thought you just wondered what it was.
I don’t think (from messing with the position and taking it apart…ish) that there is a position that it cuts off fluid flow completely. Unless maybe it’s totally fubarred.

 

[Sgt Gus]

I’m sorry I don’t understand what you’re saying. It was/is a problem?
I thought you just wondered what it was.
I don’t think (from messing with the position and taking it apart…ish) that there is a position that it cuts off fluid flow completely. Unless maybe it’s totally fubarred.

Gotcha