You're not going to truly and safely "fix" the brakes by swapping to a GMT800 master cylinder that's not really compatible with our brakes, but that doesn't stop people from doing it.
SO true.
If your '88 came with JB5 brakes (if it came with a 350, it likely has JB5),
My '88 has a 5.7L LO5, but still had the crappy JN3 brakes.
So far as I know, '88--early '1990s (perhaps '91 or '92)
regular-cab 1500 pickup trucks had JN/JB3 brakes,
extended-cab 1500 trucks had JN/JB5
unless ordered with an upgraded brake system, perhaps as part of a "towing package".
At some point in the early 1990s, GM realized that the "3" brakes (JN3, JB3) were
garbage, and they quit offering them. The "5" brakes (JN5, JB5) became the basic "power" brakes even on regular-cab 1500 pickups.
I've never seen a GMT400 with manual brakes, but it's option code JN1, and--I guess--is just the manual version of JN3.
your best bet is to adjust the rear brakes as close as you can, flush the brake fluid and bleed it, and install Raybestos Element 3 pads. My '89 has nice JB5 brakes. They'll never be comparable to my Mustang's brakes, but they're good.
If you want to upgrade to brakes that feel like a modern sports car, call Wilwood and prepare to set your wallet on fire.
The most cost-effective upgrade is to use the booster, master, front calipers, front rotors, and front hubs from an extended cab truck or other JN5/JB5 donor, replace the seized park-brake cables, and USE the park brake OFTEN, so that the horse-crap 254mm (10") Leading-Trailing shoe rear brakes stay in adjustment. If you buy the complete front brake system from the Treasure Yard, it's a downright bargain even if you have to replace some of the "used" parts--at least you can turn them in as cores.
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Better still is to use all that JN5/JB5 stuff, and upgrade to the 11.x Duo-Servo rear brakes. Easy on a K1500, harder on a C1500. Just get a "light-duty 2500" 6-lug axle of the correct gear ratio, swap the entire axle brake-drum-to-brake drum.
This gets you "JN6/JB6" brakes. Works great on a 1500.
Otherwise learn to live with the fact that GMT400 vacuum brakes are just, okay.
The biggest problems with GMT400 vacuum brakes are:
1. The entire JN3/JB3 brake system. Wimpy booster, small master cylinder bore, small caliper piston bores, thin rotors and the horrible 254mm leading-trailing rear drums.
2. The horrible 254mm leading-trailing shoe rear drums also infecting the JN5/JB5 brake system. But at least the "5" brakes are adequate up front.
3. The under-engineered RWAL system, in particular the later aluminum-bodied ones that don't have the bleeder screw that the iron ones did, and therefore need a scan tool to bleed properly.
4. All the other ABS systems that need a scan tool to bleed properly, which usually means they've
never been bled properly since the truck was new, decades ago.
The JB6 brakes--properly bled, with decent friction material, return springs, properly adjusted, etc (in other words,
working like it's SUPPOSED to) is perfectly adequate on a 1500. It's what these trucks should have been equipped from the beginning.
JB 6 brakes are what the 1500 Burbs came with, and would be a great upgrade for the 1500 pickup... ...No disrespect meant to all the 1500s out there, but your trucks ' stock brakes need help.
Yup. The 1500 pickups had marginal ("5") or terrible ("3") brakes unless specifically ordered with improved "optional" brakes. But the JN6/JB6 are fine.