No line pressure to rear disc conversion, no prop valve

sctaylor · Aug 23, 2024

88 3+3 long bed 3/4t, 2wd, 14b ff
cadillac calipers, 1/4" single line direct from Master to Tsplit hoses on rear
3/16" direct line from master to Tsplit on factory disc front.
All new except for pedal & rod.
Bleeding & getting caliper movement & pressure from front lines, but little line pressure & no caliper reaction in the rear.
Stock replacement proportioning valve was leaking out the switch hole, so bypassed the valve to see if I'd have rear brakes. Still nothing. What am I missing? I get fluid flow & pedal gets firm, but no caliper action & almost no line pressure.
TIA

nvrenuf · Aug 23, 2024

Do you still have the load sensing proportioning valve in the rear? (at the top of the rear axle flex hose)

SquareRoot · Aug 23, 2024

How do you know you have fluid flow?

sctaylor · Aug 23, 2024

Load sensing valve removed.
Getting fluid from rear calipers when I bleed system w/a vacuum pump. When using help & brake pedal tho, no piston movement.

Turbo4whl · Aug 23, 2024

Welcome new member!

You need to read some of the braking system problems and solutions discussed in these forums.

Wrong brake master cylinder made for front disc and rear drum?? 1/4" line to the front disc, 3/16" to rear drums. Front disc needs more flow, rear drums need higher pressure modulated by the master cylinder and the proportioning valve you don't have.

sctaylor · Aug 23, 2024

I have been some, thx. I suspected I may be up against mismatched parts but wasn't sure. Factory replacement lines came as 1/4 to the rear & 3/16 up front. Will a disc/disc valve do the trick?

Ricko1966 · Aug 24, 2024

Do you have the correct master cylinder? List all the changes you've made to the brake system,what parts have been replaced,added,deleted substituted. And if they were for a disc,disc application.

nvrenuf · Aug 24, 2024

Not having a proportioning valve is not the problem, I don’t have one and my brakes are fine. It’s not the right thing to do and I’ll probably run over a bus full of nuns but I’m just saying that’s not it.

I agree that you may need to look at the master cylinder. With my disc swap I’m running a master cylinder from a 99 C3500 (vacuum disc/disc application) and it works 4 D60 calipers just fine. This m/c probably won’t work for you though as I’d guess you have hydroboost.

sctaylor · Aug 24, 2024

Ok, truck as listed above, stock (as far as I know.. it's an old county highway dept truck, so all the smog stuff is cut off & gone. )
I replaced the stock vacuum brakes (disc/drum) w/all new parts & did a disc conv on the rear (brackets, caddy calipers, hoses). Eliminated the load valve under the bed. Relocated the proportioning valve to horizontal under the master (new, but stock).
Then, when couldn't get rears to act right, bypassed the p valve AND the 3-section (new ss) line to the rear & ran a dedicated single line from master to rear axle t, just to eliminate possible crud in line. (This project has been on-off for a while, as life happens.)

sctaylor · Aug 24, 2024

Ok, truck as listed above, stock (as far as I know.. it's an old county highway dept truck, so all the smog stuff is cut off & gone. )
I replaced the stock vacuum brakes (disc/drum) w/all new parts & did a disc conv on the rear (brackets, caddy calipers, hoses). Eliminated the load valve under the bed. Relocated the proportioning valve to horizontal under the master (new, but stock).
Then, when couldn't get rears to act right, bypassed the p valve AND the 3-section (new ss) line to the rear & ran a dedicated single line from master to rear axle t, just to eliminate possible crud in line. (This project has been on-off for a while, as life happens.

sctaylor · Aug 24, 2024

Dunno why it double posted, nor how to remove the repeat.

sctaylor · Aug 24, 2024

I have a disc/disc p valve ordered. Guessing from nvrenuf et al that I'm probably looking at another new master, but for a disc/disc truck?
... I read through I don't know how much stuff on 14b disc swaps before I started this, & never came across anyone (who was trying to sell the convo kits) saying new/different master was necessary. Urgh.

nvrenuf · Aug 24, 2024

My current build has a dis/disc master cylinder but my old truck had a dis/drum master cylinder, I just swap to one with a larger bore and reservoir.

sctaylor · Aug 25, 2024

So it's the master causing my problem, not the pvalve? Or is it just the reservoir size? Are the cylinders different also? Thx
Just read yours again.. bore and reservoir. Gotcha thx again.

nvrenuf · Aug 25, 2024

I don’t know that the m/c is definitely the problem but calipers use more fluid to move than a wheel cylinder, a lot of people change the m/c during the conversion.

No line pressure to rear disc conversion, no prop valve

sctaylor

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