I'm stumped...Yet another brake problem

[Madbomber88]

Ok, I'm stumped and need help with my 72 K20.

When driving on the highway, it will start to studder (can't find any coincidence as to what causes it, it isn't due to pressing the brakes), while it is doing this, if you hit the brakes, the whole truck shudders like crazy, and it feels like there's no vacuum in the brakes. Then after 40 seconds or so it goes away.

So, I just put new rotors, calipers, pads, stainless steel brake lines and hoses, and a new proportioning valve from GMC Pauls. None of that worked.

The rear is a brand new 14bFF, with all new brakes (still drums) back there too. Before I did all the work to the front, if it was acting up and I stopped, the rear hubs were really hot, but is that just normal? Would a problem with the rear brakes cause the truck to shudder so violently?

Any ideas?

I'm going on a camping trip in two weeks and have a 5 hour drive, I need to solve this.

[truckdude239]

sound like the Rear brakes are adjusted 2 tight and heatting up but this shouldn't cause any shudders. sounds like the vac booster is either bad or not getting enough vacumm

[67ChevyRedneck]

Did you install new wheel bearings when you installed the rotors? If you jack up the truck can you make the front wheel wobble? My other guess would be worn ball joints or tie rods. That should at least give you something to look at anyways.

[67ChevyRedneck]

Originally Posted by truckdude239

the rea brak ond lkthyare

Been drinkin tonight bud?

*just messin with ya

[centsless]

The rear hub may be warm, but really shouldn't be hot. Check the rear drums for discoloration ( blue ) or hot spots/ cracks. The rear brakes could cause the shaking, but it could also be loose front end parts.

[Madbomber88]

The front end is tight, and the bearings are in great shape. The weird part is that it just comes and goes. Could the master cylinder be causing it? there's only so much left to replace. I don't understand how when it happens and I press the break, there's no vacuum and the shuddering gets so violent, the truck feels like it is going to break apart!

[centsless]

Is it possible the brake pedal is sticking?

[Geezer]

That sounds like badly warped rotors but shouldn't be if new... check them with a dial indicator because something ain't running true or pressure ain't releasing from the brakes. I had a similar problem on a 92 suburban caused by the brake hoses. the inner part had torn and made a flap that would allow pressure to be applied but wouldn't release... That was a PITA to figure out... Just because somethings new don't mean it can't be defective... Quality control ain't what it use to be....

[dwaite72lnghrn]

A couple of questions. Is it a shake in the truck or a stutter like engine missing?
A shake could be a universal joint or tires out of balance. Check the joints and rotate the tires and see if it changes anything. Out of balance tires will be speed related meaning at say 40-48mph it does it and again at 60-65mph leave off speed and it will stop or lessen the shake. Universal will shake badly in a coast condition and clung when you step on gas or brake.
Engine stutter could be alot. Do you have a vacuum gage to see if you really have a vacuum lost.

[jacobs]

I'd replace the rear brake return springs and the self adjuster springs. If they get overheated, they will self adjust up too tight and cause overheating. I know, been there done that on a new 1969 c20 in 1969 pulling my 1945 Jeep around in the mountains.

[cdowns]

i'd check the emergancy brake cables to make sure they're completely releasing all the way and also try bleeding the brakes when they're hot right after use

[72gord]

Is this a 4 wheel drive? I had one that had a terrible shake that only happened once in a while and got worse when you tried to brake, it turned out to be broken rivets that held the front spring perches to the frame behind the front bumper, it doesn't explain your loss of vaccuum, but is just something else to look for. Good luck, let us know what you find!

[Madbomber88]

Could it simply be the rear wheel cylinders? The engine is fine, it just has to work hard to keep the truck moving when it happens. But would any of these possible causes just happen without being related to hitting the brakes?

[Madbomber88]

Would any brake parts that are located at the wheels cause the feeling of no vacuum, or does that mean it must be a problem with the master cylinder or booster under the hood??

[OhioDan]

Have you checked the brake pedal freeplay? What about the length of the rod between MC and booster? (assuming you might have replaced or repaired anything?) Not enough pedal freeplay, will cause plunger to booster to stay engaged causing booster to not reset allowing vacuum to be low or none. Rod between booster and MC could be adjusted to the point to where MC will sometimes not release? Just something else to check, before ya start laying out the bucks for a new setup.

[PanelDeland]

I had a Ford(Fouled-up old run-down dodge) that would shimmy ocassionally.It would just start suddenly and heavy braking to almost a halt would stop it.I had it in the shop twice and new tires and wheels.I finally found the problem.The steering box was loose to the frame.Tightened and replaced the missing bolt(1 of 5) and solved the problem.

[truckdude239]

Originally Posted by 67ChevyRedneck

Been drinkin tonight bud?

*just messin with ya

the batteries in my key borad died and i didn't look to check the spelling i did think about wraped rotors i have get new rotors before that where wraped