Hello, I just bought a 98 chevy truck with 91k miles and everything is like new except rear brakes grab and lock up easy even after cleaning, adj, greasing sides of pads, etc. and nothing seems to work.  I adj. tighter and looser and stays the same. It has new looking shoes and springs look new and all that other stuff.  Does anyone have any suggestions on what might cause this or is this a problem with the older Chevy trucks, it is a 4x4 z71.

I'd flush the brakes completely, first (Pressure ball is the best way, but can be done with the old 2 person "pump the pedal" method, just takes awhile). Double check that your front calipers and hoses are in good working order, seen sticky front calipers and soft hoses cause it before on UHaul trucks about the same year.

Other than that, he's right, can only be the master cylinder or possibly just the check valve or worst case, the vacuum booster itself 

Ray

I will hold off. I was thinking master cylinder but this is not the typical master cylinder or booster problems I have seen on other vehicles. Thank you for the info.   

Ok, I'm assuming this is a K1500, 6 lug setup right? 

First off, the reason I said don't bleed it or replace the master cylinder is because we've got a problem of the rear brakes working too good.  Hydraulic parts/systems cannot wear out and work better than they did before.  Not possible.  You can't make too much pressure with a bad master cylinder.  I'd ask how the pedal feels, but it's perfectly normal in these trucks to be able to push to pedal to the floor at idle, so that's not a good indicator.

The problem is most likely the rear shoes themselves or more specifically, how and where the shoe contacts the drum.

Make sure that the leading and trailing shoes are in the correct locations....the shoe with the shortest/smallest lining goes towards the front of the vehicle.  If the linings on all 4 shoes are same length, see if they are positioned differently on the steel, and see if front shoe has the leading edge of the lining higher or lower than the rear shoe.

Modern brake shoes are a crap shoot because the days of measuring the drum and then arcing the shoes in the repair shop on a cam-grinder to properly fit the drum are long gone...we're dealing with one size fits all shoe arcs, that are usually not exactly right. 

The most important factor in a drum brake system is where top of the leading shoe lining is located relative to the wheel cylinder and lower anchor point.  The farther toward the bottom you move the top of the lining, the more effective and "self actuating" they become, becuase you're increasing leverage on the contact point as well as improving the geometry of the system.  Conversely, the farther up towards the wheel cylinder you move the top edge of the leading shoe lining, the less aggressive the brake becomes.   A lot of the shoe relining plants have gone away and most product lines have been using brand new shoes from mostly China...which is fine, if the factory understands the concept of how brake shoes work, which many don't....many started off making brake pads, where higher mu (friction coefficent) is generally better....with a shoe, this is not the case.   Not only do you have to use the correct length lining, bonded to the shoe in the correct location, but also use a material that will work correctly based upon the above two conditions for that specific part number....and lots of these factories haven't been "trained" yet as to how to build shoes correctly.   

Pull a drum off and post of picture of the leading shoe, trailing shoe, as well as the whole assembly.  I can probably do better after seeing these pictures.

Also, see if you can get a casting number off of a wheel cylinder...I can run that and see if it's got the correct size cylinders on it. 

I will take pics of them and post them in a day or so. I meant to call you back today but was just too busy.  Thank you for the info. and I will keep ya posted. It is a k1500 Z71 and 6 lug.  The pedal feels normal and it does make sence that worn parts dont make better pressure or work better.  I just bought the truck last week and my 2 wheel 97 c1500 never had this problem.   

Also make sure the axle seals are not leaking axle fluid on the shoes. This will cause problems as well.

This was my first thought too. I was also thinking bad prop. valve. Rare, but they do occasionally go bad. It is also possible someone could have put the wrong master on it since you say you bought it used. If they put 4 wheel disc master it could cause the same problem. But I think lbhsbz is probably right about the shoes being installed wrong or of improper manufacture (very common).

Here is a pic of the Right Rear, longer shoe in back and no fluid leaking from either axle seal or wheel cylinders.  everything looks right it is just the right rear is really grabby.  Master has not been changed but this morning when I left for work the ABS light was on and it did not shift out of first gear till the ABS light went off about 1/2 a mile down the road.  hmmmm......

I did not measure it when it was off dang it.  The wear on the shoes are even  but I do have to use a hammer to remove

ALDATA shows a bulletin #02-05-25-006B  ck this out it does apply to your truck hope it helps has to do with low speed and abs good luck


Harry

That applies to the 2WD only.  The 4x4 has a unit bearing type hub with the ABS ring and sensor enclosed into the hub, not external like the 2WD trucks have. 

Thank You Pat...
I will give them a try and messure the drum before I go get them.  I was stumpped because nothing looked bad and it was not the usual problems I usually see.  I will keep you posted of the repair and let you know if there was any change.  Like I said it seems mostly on the right side and if that is grabbing more it would be the problem.  Once again Thank You for your time and kowledge.