Discussion in 'Axles + Brakes' started by SonnysSweetTea, Oct 17, 2019.
That's the exact route I'm fixing to go. Run the old style lines, a proportioning and call it a day.
Yea, it's definitely something in the proportioning valve then if you put the old calipers back on and get the same response. Sorry about the headache. I know it's aggravating. But you really don't need abs with our trucks. They're so light they stop on a dime. After I did the rear disc conversion it doesn't take nothing to go from 60mph to a full stop with out locking up the wheels.
Two pages, not one word about quick-take-up master cylinders or low-drag calipers.
Failing to use a quick-take-up master cylinder when you have low-drag calipers results in a low pedal that can't be fixed...until you install a QTU master cylinder.
Do you even know if your calipers are regular-style or low-drag? How about those eight (!!!) master cylinders?
"Regular" and "Low-drag" calipers with the appropriate master cylinders were both used on GM400 trucks.
You have RUBBER brake hoses...right? Have you pinched the rear hose to see if you can isolate the problem to front vs. rear? Have you pinched the front hoses?
Have you VERIFIED that the rear brakes are adjusted properly? Kicking the park brake a couple of times is unlikely to adjust the brakes if they were as far off as you claim.
In the end, it would not surprise me to discover you've got faulty ABS hardware, especially since this problem started right after you worked the ABS several times. But you've got more diagnosis to do before you get to that point.
How does a person establish the low drag and quick take up? I've never seen that labeled when looking for parts.
The rear of the QTU master cylinder has a "step" in the casting, that accommodates a huge rear piston. The 1" bore master has a 32mm rear piston. The 1 1/8 bore master has a 38mm rear piston. (I think. I may be off a bit.)
No promises on the GM800 masters. Never held one in my hand.
Calipers are harder. I have low-drag calipers on my K1500; when I pulled them apart I didn't see anything that caught my eye. When these first came out, low-drag calipers had a wider, tapered groove for the square-cut seals. My calipers didn't seem to have a wide groove, though. So maybe I'm just not seeing the difference, or maybe GM has changed the design.
HOWEVER they work, low-drag calipers MUST have a gush of fluid (high volume, low pressure) when you first push the pedal. The bigass rear piston is what provides that gush of fluid. That pushes the disc pads up to the rotor, then the high-pressure section of the master cylinder takes over and actually squeezes the rotor. If there's no big piston to provide that gush, the pedal goes way down because the high-pressure section of the master has to supply the large volume of fluid.
Oh, yeah. Forgot to mention. I converted my front brakes on my standard-cab K1500 to the more-powerful brakes that come on an extended-cab. The caliper pistons are bigger. Bigger pistons require a larger-bore master cylinder, or the pedal goes way down.
My truck came with a 1" bore QTU master. I have to convert to a 1 1/8 bore QTU master, to handle the larger fluid volume needed by the bigger caliper pistons. I got the calipers, master cylinder, and bigger booster all from the same Treasure Yard donor truck. The bigger, extended-cab parts are what's in the photos above. (Truck came with JB3 (?) brakes, and I'm installing JB5 (?))
You start playing games with brake parts, and pretty soon you're in deep shit and changing everything.
Does the O.P. have the correct master cylinder bore to go with his bigger calipers?
I understand entirely what you're saying, the biggest issue is no one states this on any of the "quick brake upgrade" posts and throw people into the problems blindly. You have to go 15 pages into a post to find this kind of information or get lucky someone like you responds. i already figured out the problem 2 days ago, put brand new OE style calipers back on and it stops on a dime again, i have a master cylinder from the gm800s that has the smaller 36mm bore all the way through setup for disc/disc brakes. Obviously that doesnt help my braking performance at all just makes a harder pedal which is that key word "pedal feels better" everyone claims from the m/c swap. i could careless, im gonna try a m/c from an 05 that went back to disc/drum setup and see how that goes, but as for the soft pedal i had is exactly what you said, my master had a smaller bore and the bigger piston didnt seat enough causing long pedal travel. to answer your first response, yes all the adjustments have been verified on my rear drums, the lines are all new and have pinched off lines to find where we were going wrong. My only question is that the gm400 master cylinders primary port closest to the booster is your front brake line, and the secondary port is the rear line. Looking at diagrams from the 99-07 abs modules its reversed, it shows the primary port going to the rear lines. Would it make a difference switching my lines to match that of the gm800s along with the 05 m/c?
Easy solution: DON'T buy a master cylinder intended for a vehicle that's set up differently than yours, when you can get one that fits properly. There's no magic in the GM800 master cylinder.
Pretty much what ive gathered is all the upgrades suck. As far as Oem style m/c theres a 1" and 1-1/8". Im assuming mine already had the bigger of the two being an extended cab.
Good info from Schurkey.........
My thoughts: improper rear drum brake adjustment would cause low pedal as you describe. Drum brakes WILL self-adjust. but only after they get fairly loose, and the self-adjuster mechanism is set up so that it only adjusts when going in reverse.
Anyway, I have been using this method to adjust drum brakes for almost 50 years, and it works perfect....... carefully mount all hardware and then rotate adjuster until you can just get the drum onto the hub. Rotate the drum by hand a few times to make sure it's free, then (using adjuster tool or flathead screwdriver) tighten the adjuster (through the slot in backing plate or drum) until the drum is locked. Now slowly back off the adjuster until the drum rotates freely with just a tiny bit of drag. This ensures the shoes are positioned where they need to be.
Good luck. Please keep us posted. I'm curious to see what your solution ends up being.
My soft pedal was a combination of the 3/4 ton caliper upgrade requiring more fluid to push than my master cylinder could deliver, and my rear shoes being completely thinned and sitting off the wheel cylinders. With everything replaced and ive already adjusted them properly. its different than what you think, mine doesnt have a slot in the backing plate to get the adjuster and its not reverse on mine, it works off the parking brake pushing a lever and then normal brake to spin it. No problems there its been corrected. My only issue is i have the gm800 m/c and while yes it has a firmer pedal than these trucks ever had, it requires way more effort to stop the truck and activates the abs quicker than i want it to. Im probably just gonna switch back toa brand new gm400 m/c and not try upgrading it again unless i got solid axle and large brakes.
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