Please help me make a decision

[Rusty Nail]

That's a good tip. Teeing from that vacuum line is a no-no for certain.
Id imagine it happens ALL THE TIME and certainly requires a manifold vacuum signal (not ported).

I have sourced running problems to weak vacuum signal resulting from end user added tees. Be it transmission modulator OR vacuum advance.

A good tip indeed.

 

[topper]

Try adjusting the vaccum modulator. Check fluid level to start

That's a problem. The hold down bolt is a little bit loose maybe a full turn would be all it needs but I can't get at it. Being disabled I have a hard time getting under the truck in this cold on the driveway so I did get the neighbor kid to replace the modulator hose for me , I checked it and it is on both ends but for all I know the modulator is bad but to get at it i'd have to pull the tranny and that's not going to happen til spring at least.

 

[Rusty Nail]

Huh?
All you need is a 5/16... Maybe a 1/4" socket? A three inch extension cant hurt.

 

[topper]

Huh?
All you need is a 5/16... Maybe a 1/4" socket? A three inch extension cant hurt.

even the kid had a hard time reaching the modulator. It sits in an awkward place you need to drop the tranny to really access it. It is not possible to replace it while it is in the truck. It is a 1/2 inch bolt so maybe were talking about something else.

 

[SquareRoot]

even the kid had a hard time reaching the modulator. It sits in an awkward place you need to drop the tranny to really access it. It is not possible to replace it while it is in the truck. It is a 1/2 inch bolt so maybe were talking about something else.

Uhhh.....nope. No need to drop anything.

 

[75gmck25]

The TH350 is probably the correct transmission. My ‘75 K25 with Camper Special package has a factory TH350. In later years they switched to using the TH400.

The K20 pickups usually had 4.10 rear gears, but I thought they used a lower ratio (numerically) in Subs and Blazers.

 

[82sbshortbed]

I recently put a new Trutrac posi rear end in with 3.73 gears. With th350 behind a 454. My rpms jumped to 2200 at 55. So I've been looking at putting a 700r4 in to add od and drop my rpms back down but still have a good pull in first. Mines also a 2wd. I was quoted for a rebuilt and with shift kit and he said have to rebuild the valve body (I think)to make it shift manually since I don't have the electric plug on mine for the trans. $2,600.

The th350 runs good so I might just go the cheaper route and change the ring and pinion gears to lower the rpms down to what they where before. Still haven't made my decision yet.

Say all that to say yours being 4wd will probably be more money than that to put the 700r4 in yours.

 

[Matt69olds]

I bet the cost to regear both differentials will be comparable to the cost of a 700 trans. Built right, a 700 will give you the best of both worlds, more low end get up and go, and lower cruising engine speed.

 

[rt66paul]

I would think that a K20 would have a TH400, but maybe the 4wds did not. The 400 is stronger than a 350, but they are strong and do not cost a months pay to overhaul. These vehicles are not known for their great gas mileage, there is little you can do(other than keep your foot out of it) to help that.
Take it to someone you trust, overhaul if needed, you can then build it for surer shifts. Keep the speed to 60mph for better gas mileage, and enjoy.