Fat brake pipe(s)

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Pinger

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Pretty sure there was no abs on trucks prior to 400's.
Skinny or fat rearward pipe?

The release of pressure to enable ABS operation thing. I've seen on a UK forum that vehicles with ABS are supposed to use a low viscosity brake fluid for this reason. Given that ABS has been in existence for decades, hard to believe that it's only being mentioned now. Pipe ID though could figure.
 

618 Syndicate

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Skinny or fat rearward pipe?

The release of pressure to enable ABS operation thing. I've seen on a UK forum that vehicles with ABS are supposed to use a low viscosity brake fluid for this reason. Given that ABS has been in existence for decades, hard to believe that it's only being mentioned now. Pipe ID though could figure.
I don't know offhand. I'll look next time I am under a square. Which will likely be tomorrow.
 

termite

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Schurkey

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1/4" brake tubing from the rear brake hose forward to whatever fittings or valving routed fluid to the rear goes back DECADES, in live-axle cars and trucks. 1960s at least, probably earlier than that. I don't know about IRS Corvettes. My '66 Toronado uses 1/4" brake tubing between the combo valve and rear brake hose.

Front brakes tended to get 3/16 tubing because the plumbing was independent LF--RF. Rear brakes were combined down the frame rail, until it split at the axle-end of the brake hose. The combined fluid path used 1/4" tubing, then split to 3/16 where it branched to the individual wheel cylinders.

The 1/4" tubing has NOTHING to do with ABS. Has to do with transmitting 2 wheel cylinders worth of fluid in one tube.

For the record, 1987 Squarebody pickups (and maybe the SUVs) had the same RWAL as the '88 GMT400. Moderately certain that '87 was the first year. Same deal with engines--the TBI small-blocks were essentially identical to the '88s. The 700 transmissions had minor year-to-year updates from '87 to '88, but I think the '87s already had the auxiliary valve bodies.
 
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someotherguy

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1/4" brake tubing from the rear brake hose forward to whatever fittings or valving routed fluid to the rear goes back DECADES. 1960s at least, probably earlier than that.

Front brakes tended to get 3/16 tubing because the plumbing was independent. Rear brakes were combined down the frame rail, until it split at the axle-end of the brake hose. The combined fluid path used 1/4" tubing, then split to 3/16 where it branched to the individual wheel cylinders.

The 1/4" tubing has NOTHING to do with ABS. Has to do with transmitting 2 wheel cylinders worth of fluid in one tube.

For the record, 1987 Squarebodies had the same RWAL as the '88 GMT400. Moderately certain that '87 was the first year.
Would be interesting to know for sure! I can't think of a single '87 I ever owned or worked on; the newest square I ever had was an '86. No wait, I also had an '89 Suburban, but I can't remember if it had any type of ABS.

Richard
 

someotherguy

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I've got an 87 at the shop, gonna mess with it at some point this weekend so I'll look and report back.
I'm gonna go ahead and bet against it on an '87 pickup. I don't recall ever seeing it on an '87, and a quick browse through some ebay ads for several 87's with really original-appearing engine compartments don't show any signs of it. All of 'em feature the familiar setup. Booster, MC, and a couple coiled lines aimed down to the frame rail. No controller, no dump valve, no combo valve, no wiring...

Now on the other hand, when you get to '90, the SUV's do have the GMT400-style skinny black box RWAL controller, and I can see the combo valve, too. Can't see the dump valve in the pics but the hardware positioning is a little different than a GMT400. '91 has the same setup. So the last 2 years of square Suburbans and Blazer/Jimmy had RWAL from what I can see. Then when they joined the GMT400 lineup in '92 they went 4WAL with that enormous unit only the SUV's got.

Look 'em up!

Richard
 

Schurkey

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I'm gonna go ahead and bet against it on an '87 pickup. I don't recall ever seeing it on an '87...
www.gmsquarebody.com/threads/rear-wheel-antilock-rwal-system-ditch-it.27673/

 

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www.gmsquarebody.com/threads/rear-wheel-antilock-rwal-system-ditch-it.27673/

That thread references a 91 burb and a 90 burb as having rwal.
 
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