Spongy low brake pedal LSPV revelation

[themorb]

Typical story, I bought an 80 with mushy brakes, upgraded front calipers and rotors to the larger 16" wheel style 1993+. Also upgraded front soft lines. Still mushy. Replaced master, bled system about a billion times..... still flippin mushy. Came on Mud, read about a billion posts, sounds like everyone else and their dog and their kitchen sink all have mushy brakes too. Then I read one little line somewhere... "bleed LSPV last". After work the next day I tried a couple pumps on the LSPV with the vacuum bleeder. Drove home... brakes are noticeably better!!!

So the next morning I'm lying in bed, sunlight coming in the window at about 6:30 am........ REVELATION! I SAW THE LIGHT!!! There is a line coming from the front brake circuit to the LSPV, but this fluid never goes to a caliper. It is the "bypass" part of the "Load Sensing Proportioning and Bypass Valve". Basically it senses if your front brakes lose pressure and if so, allows full pressure to the rears. Therefore this part of the front brake circuit can have air bubbles that never get bled through the calipers.

Anyway I guess if you read the manual and bleed the LSPV last you're golden. But most guys probably figure they know how to bleed brakes without reading the instructions.......... like me....... and then they wonder why their brakes never work.

Hopefully this post will help save a few marriages... I know my hates pumping the pedal......

 

[BlackLS1]

did you pressure bleed the LSPV? or gravity? after all 4 corners are bleed the LSPV should be the last to bleed?

 

[Flank]

Well Sonofa Bit...... Great advice.

 

[IanB]

Good to know, thanks for posting!

 

[Newps]

Anyone know how to bleed the LSPV? Can I assume it's the same procedure as a caliper? I.E. someone pumps the brakes, holds the peddle down, you open then close the bleed screw.

 

[fj4068]

Thank you for explaining this. The 1997 LC repair manual is rather unclear on the precise order, but it's all clear to me now. I did see a couple of bubbles when I bled it after the brakes.
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Newps - yes.

 

[themorb]

Correct. The LSPV has a bleeder screw just like the calipers. The manual says to do the "longest hydraulic line" first aka. passenger rear, and work to the shortest (driver front) last. And then the LSPV last of all.

I have no idea where ABS would fit into all of this -- my truck doesn't have it

Anyone know how to bleed the LSPV? Can I assume it's the same procedure as a caliper? I.E. someone pumps the brakes, holds the peddle down, you open then close the bleed screw.