Retrofit '88-'94 RWAL Rear ABS to a '72 C/20?

[trymes]

OK, so I have done some searching here and elsewhere on the web, and this topic has come up a few times, but there seems to be nothing too definitive. More often, it devolves into a discussion between those who think ABS is "The best thing ever" and those who think "ABS is Crap".

Anyhow, both my wife and I have had a few scares during panic stops. Both occurred on sections of two-lane while negotiating a corner. One was a deer in front of us, and one was a 10-wheel dumptruck. In both situations, the rear wheels locked and caused the tail to start swinging around. After that point, our brains caught up and we started modulating the pedal and making a mental note to keep a spare pair of shorts in the glovebox.

Both of us understand that you need to modulate the pedal to keep the tires from locking in a non-ABS car and that a properly operated non-ABS system can be as good as an ABS system. Having said that, when you combine the reality of a panic situation with the facts that we regularly drive ABS vehicles and that the truck is VERY light in the rear unless loaded (rare), it sure would be nice to have ABS (my opinion). For the record, the truck has a very nice set of Michelins on it that are less than a year old. Tires are not the problem.

Four-wheel ABS is better than rear-wheel for obvious reasons, and most of the dead-end threads I find focus on trying to install a four-wheel system. However, retrofitting a four-wheel ('95 and later) system would be FAR more work than retrofitting a RWAL rear-wheel only system. In addition, it will solve my biggest concern, which is the tail stepping out during a panic stop. If the truck stays in a straight line, I am happy to pump the pedal to keep the fronts from locking.

Considering that the '88-94 C/K Trucks came with Rear-wheel ABS and that the '67-'72 trucks are reasonably similar out back (at least for the leaf-spring models like mine), it seems that a retrofit ought to be reasonably easy to accomplish.

So, has anyone done this? Perhaps contemplated this? Any thoughts?

Tom

[1970gm4x4]

not really abs but i did a 4x4 conversion on an old toyota, Summit sells little adjustable proportioning valves fairly cheap, and forseeing a disproportionate braking, i installed one on the rear line when i built the truck. Took it out for a test drive and came around a corner and a car was in the middle of the road, hit the brakes and rears locked up and i was sideways before i new what happened, adjusted the proportioning valve (basically just cuts down the flow to the rear) till i could slam on the brakes and the rear wouldn't lock up. Worked great i think it would do what you want, pretty easy to install and cheap compared to what you speak of, sounds like a pain in the ass.

found the one i used:
http://www.summitracing.com/parts/SUM-G3905/

[cdowns]

learn how to drive without panic and there is no need for ABS

[trymes]

Originally Posted by cdowns

learn how to drive without panic and there is no need for ABS

I'll work on that, but until you can keep things from jumping out in front of me, I'll have to rely on my ABS crutch.

As for the proportioning valve, 1970gm4x4, that's a good idea, but would result in increased stopping distances when loaded or hauling a trailer. In other words, perhaps it isn't as good as ABS, but the savings and simplicity might make up for it? I suppose it could be adjusted when loaded, too...

Tom
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[1970gm4x4]

Originally Posted by trymes

I'll work on that, but until you can keep things from jumping out in front of me, I'll have to rely on my ABS crutch.

As for the proportioning valve, 1970gm4x4, that's a good idea, but would result in increased stopping distances when loaded or hauling a trailer. In other words, perhaps it isn't as good as ABS, but the savings and simplicity might make up for it? I suppose it could be adjusted when loaded, too...

Tom
Posted via Mobile Device

I don't think it would increase stopping distance, just because it cuts pressure to the rear to prevent it from locking up, just means it won't lock up before the fronts do, you could still lock up all four if you pushed hard enough, just makes it equal to the front.

Just like anti-lock brakes, you could say don't allow to put as much pressure so you can't stop as fast, but the idea is to engage the brakes to the point right before they would skid, because once they skid there is less resistance with the pavement and you don't stop as fast.

with a load you would want more braking pressure in the rear because the weight would allow more braking force to be applied before the tires would skid. Probably were your pressure is now.

On my toyota mounted it to the frame in the rear, i can reach under the wheel well and turn it if i wanted to without even crawling under the truck (its 4x4 so the tire isn't up in the fender well), and you can't see it from the outside.

I didn't really like the tuning process, cause i had to accelerate to a decent speed then stomp on the brakes, which i hate doing when i don't have to, but i had to get it set to the point that the fronts and rears grab the same, thats when you'll be able to stop the fastest, like i said, with a load in the back it changes things.

[trymes]

Sounds like we're saying the same thing! By preventing one axle from locking before the other, the proportioning valve reduces stopping distances when unloaded, but increases them when loaded (because it prevents us from taking advantage of the additional traction in the rear). Re-calibrating the valve for a loaded truck, or disabling it altogether, resolves this problem when loaded.

I like this solution, but I am still curious about the ABS, if only for future reference.

Tom
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[MARKDTN]

I would be scared of it. Could you physically do it? Sure, it is probably mostly bolt-on if you had a complete donor truck. The problem is balance. GM spent a ton of money and crashed a bunch of trucks to get to where it is. It is balanced by weight distribution and other factors. It might not work properly when you really needed it. I would not do it.

[trymes]

Mark,

I understand the concern, and it's probably a valid one. Then again, your point might just mean that the system is less effective than when installed in the '88-'94 trucks, but still better (for me) than the original non-ABS system. After all, it's not like much changed in weight distribution and the like for the 2WD trucks between 1972 and 1988. The trucks got fancier, looked a bit different, and the gas tank went under the bed, but they're remarkably similar for two vehicles separated by 16 years.

Additionally, the RWAL systems are pretty simple, and are designed to work with a pickup, whose weight distribution varies widely depending on load, so I am inclined to say it's less of a concern.

On the other hand, the proportioning valve is simpler and a well-known solution. It cannot modulate the pedal 100 times faster than I can, but it doesn't require a massive project and a donor truck, either.

I suppose it all comes down to whether you like ABS, how much of a project you want, and your personal estimate of the risk involved.

Tom