Bleeding the proportioning valve?

[DonFitzwater]

Helping a friend with his 24 valve 3500. He had brake fluid contamination and replaced master cylinder, flexible lines, calipers and rear wheel cylinders. He gravity bled the system until he had fluid at all 4 corners then I stopped by last night to help bleed them. We bled the brakes starting at the right rear corner and working our way forward. After an hour and about 1.5 quarts of DOT3 we have a mushy pedal still. The new master was bench bled but we didn't do anything with the proportioning valve. I've never had a vehicle with a completely dry brake system before so I'm wondering if we should have started by bleeding the proportioning valve after the master cylinder. Any ideas? No matter how much bleeding we do, the pedal is mush with the engine running.

This truck has 4 wheel ABS.

 

[Timd32]

My FSM does list a sequence. This is a 96 book. There were a couple areas for bleeding I. Section 5.

 

[JR]

Good possibility there is air in it. Just crack the highest fitting loose, you'll know right away.

If pedal is still mushy you may have air in the abs. Only way I've ever been able to correct that without DRB is to activate the abs on a dirt road by spiking the brakes several times,then rebleed each corner again.
Of course the brakes have to be good enough to get something to lock up first...

 

[BigPapa]

What year is the truck, Don?

 

[DonFitzwater]

What year is the truck, Don?

I believe it is a 99 model. Thanks for the info guys, I'll be looking into it more this weekend.

 

[BigPapa]

Procedure for the '99 requires use of DRB.

 

[Brian Neal]

There is one sentence in the FSM that got my attention when dealing with brake fluid contamination that said ALL brake components with rubber that came in contact with the contamination must be replaced, including the proportional valve and ABS hydraulic control unit. There are rubber seals that can swell in these components, causing performance issues with the ABS. I found this out the hard way last year and am still dealing with a minor ABS light issue.