1991 gmc topkick 366 engine

I just bought a 1991 gmc topkick with a 366 engine. Most of the information tags are faded but I think it s a c70. It s been sitting for 3 years but I got it running by jump starting it fairly easy. There wasn t much fuel left in it so I added a fuel cleaner and then fresh gas. It doesn t run great but it s not running to rough either. I think it s running rich but don t know how to adjust that. The next issue is all 4 brakes are locked up. If I disconnect the brake lines it can be towed and rolls just fine, but if I bleed it and reconnect it, I can drive it anywhere because the brakes lock up again. Any help would be greatly appreciated.

Pull the air cleaner and make sure it is not full of a mouse nest or just flat out dirty. Also if it has a carb make sure the choke opens up like it should

(quoted from post at 19:16:52 01/17/23) Pull the air cleaner and make sure it is not full of a mouse nest or just flat out dirty. Also if it has a carb make sure the choke opens up like it should

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So it s fuel injected, and the air filter is clean. Well, it could be cleaner but it s clean enough that I don t need to replace it yet.

Surprisingly enough , built until 2004 .

Hello Mitchell welcome to YT! On the brakes try
removing the hose and trying to blow air through them.
I personally witnessed a 90s era Camaro that sat a
couple years. When the owner needed to move it both
front brake started locking up. The brake hoses were
the problem, the inside lining deteriorated and came
loose doubled over and acted like a check valve. It
would let fluid apply the brake but not flow back to
release them. Heard the same scenario from posters
on here since then. On the engine it should have a
..Service Engine Soon.. code light on its dash. Can you
see it when you turn the key on to start it? If so does it
go off when it is running?

That s a good idea on the brakes, I ll give that a try. Do I need to unhook the lines I m blowing out completely then I m guessing? And it doesn t have any lights that turn on after it starts up.

Plugged hoses is not unusual in modern trucks that are in constant use but on an old one that has been sitting you will most likely need all new wheel cylinders. The fluid collects moisture which works its way down to the cylinders, and they corrode and freeze up. Unless you will be working the truck frequently replace the fluid with silicone. It won't absorb moisture.

What type brake booster does it have?

If it's a Hydro Boost, or vacuum assist, be sure the master cylinder push rod is properly adjusted, and
the master cylinder piston is fully returning back to the snap ring. If it's not returning, it will build
pressure as you describe.

Also, having the wrong fluid, as in someone putting oil in the master cylinder will do that. If any rubber
components are swollen, that is why. It will need to be completely rebuilt with new hoses, MC, wheel
cylinder cups, and everything thoroughly flushed with nonpetroleum based solvent.

As for the rough idle, with the air cleaner off and engine running, check the spray pattern of each throttle body injector. There is a good chance the injectors need cleaning. I'm just doing a job like that myself right now. There are some helpful YouTubes on this very subject.

Rough idle can also be from the throttle plate not fully closing due to carbon build up.

A good cleaning will usually smooth one out, though it will be a temporary fix.

Also a leaking EGR valve will cause it.

So I disconnected the rear break lines and tried to blow them out with an air compressor and couldn t get any air through any of the lines. Im assuming I need to replace those. Im wondering if I need to blow out/replace the metal pipe sections though. Do those do the same thing or is it typically the rubber only?

(quoted from post at 17:14:17 01/19/23) So I disconnected the rear break lines and tried to blow them out with an air compressor and couldn t get any air through any of the lines. Im assuming I need to replace those. Im wondering if I need to blow out/replace the metal pipe sections though. Do those do the same thing or is it typically the rubber only?

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I had a bad rubber line on my '06 F350. The guys at the shop told me that it is not uncommon with them. However I got the impression that it was a thing with modern trucks. Before taking it apart and draining the fluid I would hold the pedal down hard for 30 seconds or so if it holds pressure so that if the steel line is leaking you can ID the leaking spots. I would Disconnect the steel line from the tee on the axle to see if it flows there. Same as checking for a fuel line blockage. Old lines are more susceptible to rust through from the outside. If you have to replace lines price out copper nickel. It is soft and bends easily and doesn't rust.

I have never seen an instance were a steel brake line
has plugged. I should have answered sooner, but I was
suggesting to just blow through the hose once it is
completely disconnected on both ends. You say you
disconnected the rear brake..lines .. and could not get
any air through them.. I am guessing you meant to say
hoses. If so then replace them. To add I would not
suggest blowing air through your brake line or for
certain not trying to blow air forward through the line
and out the master cylinder. You will most likely find
out that bleeding the brake system on one of these
trucks can be a real pain. Probably to late now but just
to help keep air out of the system when I disconnect a
a brake line I have rubber caps I place on the end of
the line to keep the master cylinder from loosing all the
fluid. This helps greatly when bleeding the brakes.
Also, getting the wheel cylinder bleeders broke loose
and open can be a pain. Once you get the hose back
on fill the master cylinder with fluid and open a bleeder
on each wheel one at a time. Give the fluid some time
to flow by gravity down to the wheel cylinders. Once
you get a solid flow of fluid close that bleeder and
move to the next one. There is a possibility that if you
let all the fluid drain out of the master cylinder it may
not re-establish gravity flow due to air locking. This
initial bleeding helps enormously when moving to
bleeding with the brake pedal. It also answers your
concern about the steel lines if the fluid flows through
them by gravity you have no worries. Also as others
have mentioned this truck may have a hydro vac brake
booster on it. The brake lines will run through it and it
will have a 10-12 inch diameter container looking thing
on it, this holds the diaphragm that the engine vacuum
acts on to essentially give you power brakes. If it has
one there is often 1 or 2 bleeders on them as well. Also
brake adjustment is very important especially on those
rear duals. There are slots usually plugged with covers
in the brake backing plates to access the star
adjusters. Usually turning the star wheel so the teeth
turn toward the brake drum tightens the brakes. This
should be done with the wheel jacked up until you start
feeling some drag on the wheel. Spraying penetrating
oil on the adjusters is usually almost mandatory.