bigbrainonbrad Reader
9/18/10 11:51 a.m.

Buddy at work has a 1968 Pontiac Catalina 2 door, 400 with a 2 barrel carb and auto. He hasn't driven the car in a few years and is just trying to get it going with the help of his brother-in-law/mechanic. Rebuilding the carb and redoing the 4 WHEEL DRUM BRAKES (OMG THAT MUST SUCK!!!). He was saying that the transmission doesn't like to shift up a gear unless you back off the throttle, my line of thinking short of a bad transmission are a poorly adjusted kick down cable and/or clogged filter limiting line pressure. Am I completely off base or do those thoughts sound reasonable?

I've been talking to him about updating the carb/manifold and the brakes. I told him that by switching to a (more) modern carb and manifold there is probably some power to be had and a good chance of some MPG's. On the brakes being a GM I suppose that junkyard upgrades exist. From what I have found it is a B body so could he go to a junkyard and grab a complete brake system from some 80's barge and swap them on or is there a better option?

stuart in mn
stuart in mn SuperDork
9/18/10 7:24 p.m.

The transmission is a TH400, so tech information or support for that should be easy to find. The factory 4 barrel carb and manifold for Pontiacs of that era were actually very good for street use on a big car; a 1967-1970 carb and manifold should be easy to find and will cost less money than aftermarket. The main benefit of an aftermarket manifold would be less weight since they're aluminum. For brakes, disks were an option starting in 1968, and the disk brake setup from any big Pontiac from 1968-1970 will bolt right on (including the spindles.) However, the big problem is they are hard to find and replacement rotors are nearly nonexistent. Also, they don't cross reference with Chevys or any other GM cars. A better choice is an adapter kit from that allows the use of easy to find late model rotors and calipers. Having said all that, a properly rebuilt and adjusted drum brake setup will work pretty good, as long as he's not road racing or driving down the sides of mountains - they had decent braking power for normal driving, but will fade under heavy use.

patgizz SuperDork
9/18/10 8:02 p.m.

th400 does not use a kickdown cable nor would that have anything to do with the shifting. throttle valve cables do however. th400's use a solenoid to provide the kickdown.

after sitting for a while i would change the filter in the trans and see what happens.

4 wheel drums function fine, as long as you do not need to do several hard stops in a row as they do fade more than discs. i have had them before, and while i do not prefer them it is mostly due to the ease of pad and rotor changes versus drums, shoes, countless springs, and wheel cylinders. i could never recommend a product from the vendor mentioned above for disc conversion brackets as the ones i purchased from them were flimsy and the calipers they spec'd did not even fit the brackets.

stuart in mn
stuart in mn SuperDork
9/18/10 8:07 p.m.

That's interesting - I know several guys who have installed the Scarebird kits on their 1960s Pontiacs and they've all raved over the quality.

Duke SuperDork
9/18/10 8:36 p.m.

My guess on the trans would be the vacuum modulator. The ancient diaphragm probably has a pin hole in it so under open throttle it won't generate enough suck to make the trans upshift. It should be bolted onto the side of the transmission and be relatively easy to replace.

BobOfTheFuture HalfDork
9/19/10 12:50 a.m.

My god, This forum is amazing.

porksboy Dork
9/19/10 12:05 p.m.

+1 on the modulator. Also check the fluid as a torn diaphragm can suck the fluid out of the trans up the vacuum line.

Dr. Hess
Dr. Hess SuperDork
9/19/10 12:39 p.m.

And make sure the vacuum line itself is in good shape.

psychic_mechanic Dork
9/19/10 2:39 p.m.

I had 4 wheel drums on a couple of my mustangs and they do work well enough if they are adjusted properly, unless you run through deep water.

I was going down a road with poor drainage, so there was a decent amount of standing water on the road and a stop sign less than 1/8 mile away from said pond. I stepped on my brake pedal, which was still firm, but nothing happened due to the soaking wet shoes and drums. I downshifted my C4 to low and stood on the brakes as hard as I could and ended up rolling into the intersection a bit, thankfully no one was coming.

Since then if I am driving a 4 wheel drum car and I go through water that's deep enough to make kids smile I will lightly apply the brakes for a few seconds after exiting the water. It doesn't take much heat to turn the water to steam and it is better to do it when you aren't counting on the brakes to stop you.

The fact that I was a young (inexperienced) driver, driving a car with aged brake technology that I was responsible for maintaining (poorly at best) probably played a role in this as well.

Our Preferred Partners