... To achieve strong stopping power, I have to give a second pump, first pump will slow car, but second stops it.
Good function after pumping is a sure sign of something "giving". Usually it's air in the system. Sometimes it is other parts flexing or expanding, such as bad brake hoses, or flexing brackets or calipers. It is commonly a simple mis-adjustment or air in the rear brake system, causing the pedal to effectively bottom-out without the second pump.
I would first do the "other" inspections, with a buddy repeatedly pressing the brake pedal, and you under there watching and carefully feeling for bulging, flexing, or other odd stuff. Any
motion or movement means a lot in braking systems. If no joy, I would then verify all areas that air can be trapped are purged. For example, this means verifying that the internal bleeder path in each caliper bore is actually at the highest point - a common issue with swaps.
If there is a friendly shop or well-equipped car club member nearby, have them test the line pressure at each wheel to find which circuit is spongy or lazy. Try reverse bleeding if necessary, and re-bleed your MC if nothing else is getting it. Remove the calipers, insert a block, and bleed them at various angles if necessary. Analyze the system carefully and get creative! Sometimes, this is like "Where's Waldo?", and it will be obvious when you finally 'see' it. Have patience - you'll find it.