where to get advice on 86 G30 front disc brakes?

[Mr. Bone]

I am looking for advice on Chevy front brakes for an 86 G30 chassis. The brakes gradually lock up while driving. I replaced the calipers, lines, and pads. Is it possible that the reman. calipers are faulty because after the changed the calipers it worked great for about a week. Then all the sudden they locked up again. thank you.

[Robyn]

I would be looking at the master cylinder.
Someone may have installed the wrong cylinder or have installed a residual pressure valve on the disc brake side of the cylinder.
The residual valve is used exclusively on the rear brakes with wheel cylinders to keep the cups expanded in the cylinder. The Disc units use a square cut seal that allows the pads to retract just enough to be free but not all the way back.
Check your master cylinder to see if the port feeding the front brakes has a residual valve and if it does remove it.
Have you changed the master cylinder recently??
Let us know and we will go from there.

Robyn

[NH2112]

Sounds like a master cylinder issue. There are valves in there that keep the lines "preloaded" with a slight amount of pressure so that pressure doesn't have to build up from 0psi every time you hit the brakes. If they stick, too much pressure will remain in the lines.

[rustyk]

I agree - it sounds like a pressure leak (I assume you are cursed with the brilliant HydroBoost system) which is applying the brakes gradually. Master cylinder...

[Mr. Bone]

Hello All,

Okay . . .it's done. I replaced the MC and things are so far so good. Thanks for the advice. Its only been 24 hours though and the van acted fine after changing the brake lines, pads, and calipers on previous occassions so I'll keep my fingers crossed.

What is the hydro-boost system anyway - still having a hard time finding literature on the van so my knowledge is limited.:confused:

thanks again.

[DmaxMaverick]

......(I assume you are cursed with the brilliant HydroBoost system) which is applying the brakes gradually which is applying the brakes gradually. Master cylinder......

Cursed???

The Hydroboost system has been used on GM (and other mfg) vehicles since ~1980, and is essentially unchanged since the initial design. Although there are issues with it, and failures do occur, it has proven over the years to be a very reliable system. Much better than a vacuum alternative. It is possible, but I've NEVER seen/heard of a Hydroboost applying pressure to the MC w/o pedal input (this could be the first--stranger things have happened).

[rustyk]

My '89 G30-based motorhome had Hydroboost, and on my way home after taking delivery, the P/S belt broke, leaving me with no power steering and no power brakes - I made it home (only about 4 miles on lightly-travelled roads) without incident.

My '94 DP motorhome (6.5L TD) also has Hydroboost with a serpentine belt. In Alabama, the A/C compressor locked up and broke the belt - long downgrade curve, no power brakes, no power steering on a 16K lb. motorhome - not the most fun I've had.

What I don't like ("cursed") is if there's a problem, one loses boost to both steering and brakes.

Only once have I encountered a power system (vacuum) where the master cylinder had a leak that seemed to allow the brakes to be applied lightly - at least that's what we concluded. Swapping the M/C out solved it.