Prop valve/hydroboost question

[loganobrien121]

Few weeks ago finally got around to installing the front discs and hydroboost setup. Also, new drums, shoes and wherl cylinders on the rear.

Driving it around it would lock up occasionally- but only on really sudden stops. And honestly- the braking isn’t THAT much better than the drum/drum i had previously. The pedal is much stiffer but I really don’t feel that it’s leaps and bounds better than where it was before...

Anyway Flash forward to today: was going up to the mountains and on the way, I had 2 instances where it locked up real bad during moderate braking.

Then, a bit later, on a pretty steep decline, I lost braking power. Felt like I lost power assist. Kind of a **** your pants moment. The rears So I turned the prop valve down a bit. Seemed to make it all a bit better. But still locking up a little bit.

Should I just yank the prop valve down and then adjust from there? A bit scared to do anything drastic but I can tonight when the roads are empty. Or does this sound like something else other than the prop valve?

[SJTD]

I'm not clear on what's happening. You said it locked up twice. Front, rears or both?

Since you have an adjustable prop valve it should be set so the fronts lock first. Ideally they'd lock at the same time but since that's impossible to achieve since loading and maneuvering will affect braking so the fronts are set to lock first.

The booster working or not wouldn't make a difference on which locks first.

I'd think the way to do it is to set the valve where the fronts are locking then add some rear and test. Repeat until the rears lock first then go back the other way again.

[loganobrien121]

Okay i’ll do that then- the rears were locking up, not the front. I turned to decrease on the prop valve a little but I think I need to do more.

What I’m kind of more concerned about even though is that when my rears went out/faded on that decline, it was hairy... it felt like the fronts weren’t doing that much- is that something to do with the prop valve needing adjusted? Still not 100% sure how it works- been reading about them today but I wasn’t able to find an answer: if too much is going to the rear does that mean less is going to the front?

[SJTD]

My understanding of how the prop valve works is that the fronts get full pressure always. The pressure in the rear system pushes the piston in the prop valve against its spring which absorbs some of the volume going to the rears. The weaker the prop valve spring setting, the less pressure to the rears.

If the rears were locking you're loosing a lot of braking power. Since the fronts can do most of the braking and you presumably let up when the rears start to lock you have a very ineffective system. If you don't let up you're asking for a spin.

You always want the fronts to lock first. Since a skidding tire has no lateral control, if the rears lock and the fronts don't the rear end is going to come around. An analogy would be balancing a pole on your finger compared to letting it hang downward.

It seems to me that as the rears faded braking should have actually improved. They would have gotten weaker allowing you to apply more pedal pressure making the fronts work better.

What prop valve are you using? Does it have only two ports? Plumbed in line to the rears? Original prop valve removed?

Mine has a 3/4 ton Ford front with Lincoln rear calipers and, I think, a Wilwood prop valve. Unfortunately it was only on the road once when I towed it a few miles from where it was to where it is.

[cmaje72]

What master cylinder are you running?

[Cheap Hobby]

What master cylinder are you running?

Good question. If the m/c was included in the kit is from a corvette the resivoirs are reversed. If you installed your brake lines in the same position as the stock m/c, you are sending front brake pressure to the rear.

[SJTD]

Does a Vette master have a stepped bore?

Unless the pistons in the master are different sizes the pressure would be the same.

[Cheap Hobby]

Apparently they do. Everything I have read and every one I've talked to has had to swap the position of the lines when using vet m/c no matter what they put it on.