If you clamp the front LEFT hose, does it also improve as much? How about the rear hose?
Perhaps the caliper piston is not sliding properly on the square-cut seal (old seal, damaged piston) or the caliper isn't sliding on the mounting pins/sleeves.
Might be time for a replacement caliper.
Reverse-bleeding can be done, it works, but has three caveats.
1. DO NOT push dirty fluid backwards through an ABS unit. Generally-speaking, the most-contaminated brake fluid is found at the lowest points in a hydraulic system--the wheel cylinders. You MUST flush the old fluid out in the usual direction, or start with a clean, empty system before forcing clean fluid UP to the master cylinder.
2. Don't over-fill the master cylinder reservoir. At some point, you'll probably have to suck some fluid back out of the reservoir or it'll overflow.
3. The bleeder screw is higher than the brake hose/brake tube connection on the caliper/wheel cylinder. Which means, there's likely going to be a small air-bubble trapped in the wheel cylinder/caliper that will not bleed back to the master cylinder. So the LAST thing you do when "reverse bleeding"...is to bleed in the normal direction to assure that all the air is out of the wheel cylinder/caliper.
So here's the process:
1. Flush the fluid in the normal direction, to remove all the contaminated fluid.
2. Reverse-bleed the system with clean fluid.
3. Bleed in the normal direction just a little, to assure there's no tiny air bubble between the bleeder screw and the brake plumbing.
By the time you do all that...you "probably" got all the air out in Step 1, making Steps 2 and 3 unneeded.
You've invented an inexpensive plastic-bottle reverse-bleeding tool. There's at least one company (plus twenty Communist Chinese bottom-feeder knock-offs) selling unreasonably-expensive "special tools" for "Reverse Injection Bleeding".
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But in the end, all you need is...the disc brake caliper. (Doesn't work well with drum-brake wheel cylinders...sorry.)
Pump the brakes so the piston is 2/3 the way out--not far enough to push it past the square cut seal.
About like it would be if the pads were worn-out. Bleed the brakes in the usual way, so the caliper now has clean fluid inside.
PUSH THE PISTON BACK IN. You've just shoved almost the entire caliper's worth of fluid back through the plumbing, up to the master cylinder...taking any remaining air with it.
My friend's son was doing the brakes on his Avalanche. Installed rebuilt calipers. Bled the brakes several times, had a low-squishy pedal. He calls his Dad...who calls me. I asked if the bleeder screws were facing up or down.
Yup. Left caliper installed on the right side, right caliper installed on the left side.
Good call. Given enough weeping, you might see the dust-boot inflating with fluid. If the dust boot were CAREFULLY pulled away from the piston, (do not tear the dust boot) the fluid would leak out.