So I finally had the chance to replace my driver side front calipers, rotors, pads, UCA, and axle. However I only did the drivers side. The passenger side pads are wearing thin.

My problem is that I'm losing pressure whenever I depress the brake. It doesn't bite and it actually drops all the way to the floor at times when pressing the brakes slowly to a stop. It makes a hissing sound when flooring the brake and releasing it quickly.

I bled it, but only the front driver side. The fluid was still a honey color bc I was in a hurry but there were no bubbles and I repeated it maybe 10xs.

This only started after swapping to a used ebay OEM caliper change.

Any ideas what could be causing this?

 

Even if you did not have time to replace everything, you always replace pads or shoes in pairs. Different pad compounds may lead to uneven braking.
Are you positive you have the correct caliper on the correct side?
The bleed screw should be above where the brake line attaches.
When you bled the brakes did you floor the brake pedal?
If so you may have damaged the piston seals inside the MC. When MCs are older or are not bled often, there is a build up of crud where the pistons do not normally travel. Once the pistons ride on this rough surface the seals may become damaged and will no longer be able to maintain pressure. You can test this in two ways. Turn the car off and pump the pedal til the boost assist is depleted. Does the brake pedal feel firm now? If so without assist you are unable to create enough foot pressure to blow out the seal, but when full booster assist is used you are able to blow the seal.
Turn on the car and go for a drive, when coming to a stop use the parking brake to help slow you down and gently press the brake pedal. Does the pedal come back? If so again this is because you are not using the full force of the boost assist and you are not causing the seal to roll internally.
How long has it been since the MC/system was last bled?
If it has never been bled or been several years, new brake fluid does not compress and can cause the old seals in the MC to swell/weaken which will cause for an internal MC leak. And if the MC was floored while bleeding these seals are even easier to damage when swollen.
Check that there are no external leaks at the caliper, at the hose fitting or anywhere else. Touch under the MC on the booster, is it wet?
If the caliper is on the correct side, there are no external leaks, and you have floored the MC, it is most likely that the MC has an internal leak and needs to be rebuilt/replaced.

Keep in mind being a FWD the brake system is diagonally split. Driver Rear/Passenger Front and Passenger rear/Driver Front.

 

Alright thanks for all the information. Sorry it took so long to respond, I just got married.

I did floor the pedal when bleeding.

When the car is off and I try to pump the pedal, it's like I'm hitting a wall. There is barely to no depression of the brake pedal. 1) Is this how boost assist feels?

1) Can you recommend a good aftermarket BMC?
I've heard to avoid Fenco. Oreilly's is BrakeBest. Napa's Napa Ultra Premium. I can also get a Cardone Select. They're all around the ~$55 range.

Thanks again.

 

You seriously only replaced the driver side ? I can't fathom anyone in their right mind only doing one side in recorded history. Why would you only want to do one brake pad? When you buy pads they always give a set and you didn't change out both sides ?

The brakes are always hard to press when the car is off. The booster uses vacuum when the car is running to help you press the brakes.

I'm guessing you have a brake fluid leak(most likely brake line bolts when u replaced caliper) and not a bad master cylinder, assuming you had no issues before replacing the calipers.

If one side brake pad is wearing less than the other that usually means that you either need to take out the sliding pins and regrease them or the caliper itself is bad.

 

Did you have good pedel feel before? If not, then your MC is leaking internally external leak, or air in the ayatem. If you had good pedel feel before, you either have air in the system, or you damaged the MC and have internal leaking, when you backed the caliper off to get the old pads out.

 

If from your previous statements, boost causes the pedal to floor. And no boost has the pedal rock hard, then there is an internal leak of the MC.
Needs to be replaced.
Go to RockAuto, use CENTRIC premium brand(not CTEK).