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.