I bought a used cannondale bad boy ultra with drop bars and 10 speed shifter levers and hydraulic brakes.

The brakes use a cable to hydro conversion master cylinder that is held to the steering tube with something similar to a front reflector bracket.

I bought the bike knowing that it had the bad a rear brake system and that the rear brake dragged. The seller had already sold the bike and had pulled out from the sale when he noticed the issue. The original buyer wanted nothing to do with it after knowing about the brakes and the guy offered it to me for $400 less (full retail of the entire front and rear kit)

My guess was that some used mineral oil.

Hoping (but doubting) a complete bleed would solve the problem I bled the brakes according to a guide on this forum.

Before the brakes where dragging and did not engage at all besides the dragging.

Now when I press the lever down the is a decent amount of stopping power for a split second. I could be spinning the wheel with the bicycle suspended as fast as possible and it was instantly stop the wheel but then right away the wheel can move again.

If on the bike a feel a jerk and then no resistance.

Visually you see the brakes pads squeeze and then slowly retract.

My guess is I need to rebuild the rear caliper...

I have not found a guide to rebuild the master cylinder but am hoping it is fine.

I am wondering if before doing this I should swap the front and rear tubes that go to the master cylinder rebleed and then try again... My thinking is if the rear suddenly works and the front does not then it is the master cylinder not the caliper that is damaged.

Also one thing to note...

I smelled the fluid that came out... It had a strong pungent smell.

My guess is that any DOT fluid would have no odor or smell sweet. The 5.1 I replaced it with smells nothing like the fluid that was removed. It actually had no discernible smell.
Or is it possible that the fluid had an animal detractor that smelled bad...

Anyways

What's the recommendation with this.

I have a mechanical and technical background with lots of motorsports experience but am new current tech bicycles.

Also are there other hydraulic calipers that are "mostly" compatible... Possibly not knowing what they are getting into a shop quoted me $25 in labor plus the rebuild kit.

Or I can do it myself (watched a video it's pretty straightforward but everything is tiny)

If I could find a used but working or new caliper for under $75 I would just do that

Thanks

too add to things... Is it possible to run a small lighter rotor with less leverage. What parts would I need... With the majority of braking done up front I would love to save some weight in the bike while making it so that about the same amount of force on both levers makes for a decent forward balanced braking.