hydroboost + chevy disc swap problems (partially fixed - still need help)

[skrit]

Just completed a chevy front disc swap including a new brass proportioning/metering combo valve and Ford F250 camper special master cylinder connected to my chevy astro hydroboost . Rebuilt the rear drums with new self adjusters, shoes and emergency brake cables. Installed speed bleeders (with check valve) in calipers and drums. Fully bled the system.

Problem 1: soft pedal - fixed this by adding a 3/4" solid aluminum spacer in the Master cylinder so that the hydroboost push rod will engage the master cylinder piston sooner. Now the pedal is slightly hard which is OK with me.

Problem 2: my rear brakes lock up prematurely even though I added the brass proportioning/metering combo valve and installed it per this diagram http://www.mpbrakes.com/uploads/documents/optomize.pdf

Can someone please help me. At 10-15 mph a quick application of the brakes locks the rears up. I can't tell if the fronts work at all.

UPDATE 1: installed the wilwood prop valve in the rear line. reduces rear lock up somewhat but I can still hear them trying to grab even when the valve is set to minimum brake power. Front calipers alone will not even stop the truck or get locked up when I slam the pedal. Also went ahead and changed my rear wheel cylinders. BTW, those speed bleeders suck. Out of 4, 2 had stuck check valves so they wouldn't even bleed properly.

UPDATE 2: Thanks to Broncobowsher and others for helping me fix my front brakes. My bleeder screws were at the bottom. I switched the right and left calipers which made the bleeder screws be located at the top, Now ALL of the air is bled out and my brakes work like a champ!

 

[bigmuddy]

I had more or less the same issue unitl I added the LBS valve from BCbroncos and I don't think I could lock them up if I tried any harder.

Ben

 

[supermottl]

try using an adjsutable prop. valve. I have a Willwood, and i can dial it in. My setup is an Astro Unit, Vette MC, and chevy/caddy setup..
works great

 

[swa0330]

One other thing, although not very likely, is that there may be clearance needed between the knuckle and the caliper. I don't think that is the case for Chevy, but maybe. If there is insufficient clearance, then the caliper won't grab in a vertical plain and will make for weak braking. It can be slightly at a tilt and decreases the effectiveness.

 

[skrit]

bigmuddy, is the LBS valve the same as a residual valve? At $92 - it is awful expensive. I assumed the proportioning valve would have cured it. Now that it is plummed in, I don't want to have to take it out.

 

[Broncobowsher]

Sounds like you need the adjustabel prop valve. The one size doesn't fit all OEM part isn't set with the proper balance.

But do check the front caliper clearance to the knuckle. The clearance gets tighter as the pads wear. So if there was enough, there may not be enough now.

LBS is a cylinder with a piece of squishy rubber inside of it, like on a cheap mountain bike suspension. It is there to make the brakes squishy (the opposite of going to braided brake hoses) with the idea that a bump in the rotor or drum causes a spike in brake pressure that causes the brakes to lock up at that point. By adding a squish to the brake system the bumps in the brakes are not suppose to spike the brake pressure causing them to lock.

I use an inline prop valve from speedway motors. Not even one of the big fancy ones, but a stelth inline one. I have in inline where the braided line off the master goes into the hard line at the frame rail.

 

[KManuel]

Same problem, a wildwood adj. proportioning valve for the rear did the job

 

[skrit]

OK I'll get an inline adj. prop valve. I see one on eBay I'll get. I assume I can just put it in the rear hardline from my MC before the combo valve. (I don't have braided flex lines). Thanks for everone's response. I'll update you all when I get it installed. I'll probably have another problem after this....I just can't get my brakes right no matter what!

 

[skrit]

Ok so I got the Wilwood valve installed in my rear brake line (near my MC) and got my hydroboost working again (see http://classicbroncos.com/forums/showthread.php?p=956605#post956605).

After turning the wilwood valve to reduce the rear brakes, my front brakes are extremely weak and will not lock up. The rears grab a little but dont lock either (so the wilwood definitely works). Do I have the wrong master cylinder for my setup?

 

[Kenny]

Just in case, do you have the F250 M/C ports plumbed correctly? Rear port (big reservoir) going to the front brakes and front port (small reservoir) going to the rear brakes.

 

[Broncobowsher]

But do check the front caliper clearance to the knuckle. The clearance gets tighter as the pads wear. So if there was enough, there may not be enough now.

You did check this as well, right?

 

[skrit]

You did check this as well, right?

I ground the crap out of the knuckles and if I remember correctly there was plenty of clearance - but I'll take the wheels off and look again - is it 1/8" clearance that I need? Also, I do have the large chamber of my MC plumbed to my front brakes.

 

[Kenny]

BTW, those speed bleeders suck. Out of 4, 2 had stuck check valves so they wouldn't even bleed properly.

Are you using the speed bleeders on the front calipers? Maybe swap them out, and bleed the old fashioned way? Also, are the bleeders at the top of the calipers? If they're at the bottom, you won't be able to get all of the air out.

 

[skrit]

Are you using the speed bleeders on the front calipers? Maybe swap them out, and bleed the old fashioned way? Also, are the bleeders at the top of the calipers? If they're at the bottom, you won't be able to get all of the air out.

bleeders were replaced and are on the bottom which means the calipers are installed at the back of the rotor. i've seen several installs which are like this. Is this a theoretical problem or is it a proven issue?

 

[Broncobowsher]

bleeders were replaced and are on the bottom which means the calipers are installed at the back of the rotor. i've seen several installs which are like this. Is this a theoretical problem or is it a proven issue?

Swap your calipers left to right. That will put the bleeders on top. Yes it is a proven problem. Air forms a bubble in the caliper. you need to release that bubble from the top.

 

[skrit]

Swap your calipers left to right. That will put the bleeders on top. Yes it is a proven problem. Air forms a bubble in the caliper. you need to release that bubble from the top.

I was skeptical of whether the bleeders at the top really made a difference. Well I swapped the calipers and found out......IT DOES. My front brakes work great now! Thanks to everyone.