Jimny Big rear brake upgrade

Good day Team.

I have dived into a  deep project of upgrading the rear drums to disc brakes. The reason being was I am running 31" tyres and also towing an offroad/overland trailer.  The drum brakes also always wore down in mud and I am tired of changing them..  I did install power disc brakes in the front earlier this year, so I also had the stock discs that can go in the rear.

Long and short, I  am based in South Africa and I really struggled to find some Renault Megan callipers locally. (Importing was crazy expensive shipping and duties). I really wanted a new set of callipers, but just couldn't find them. Every motor parts shop said to go check a scrap yard. Admitting defeat I called about 15 scrap yards and the only option was a Renault Megan sport.  I drove there and it looked like the right calliper, cable hand brake, bolt holes all lined up etc. and a cool bonus they were (RED)

https://lh3.googleusercontent.com/rZEFp-grtPgA8gLK3RBv-cKsfJmTuo7m8As2uRb1EpYxXuuSW-Bf-UHcD4FYvPR58VuboEpfqcnRWzYZ7ZACLHTCkJQIt96JmvUuznWVHFZZI4Bll2MCsAMpRchgwGYmtLv5e9VOduEmn5iukT-f9vv4f9BWbLtqBvHT-8odH7eKguLfLh19FY7yyC8h2mTWa5ncEx1dyPaUUMUPdNe8Z64AucRBNhjUoaRin0x1bvzxx-Rswp7b20uSnQLlC5Fg1ofCK6zo47vh6IXwivfTo3i1wy_adlz9xDptHo3hIFnXblJTj3xZzdHiWOa-JB_LYHLeTW3CFwGhV7ysolI1XR1c96MGwniWZ2DGAmWgAvYT9vBhIhBhUSraplEy0F6Cmgh0eqmydtznd2sCUC6d3tfZP7L1y2-PN7tiQH6YEq9S_-h1ymNIrnz85EfWuobmoVmSh3C7hVjgIq51Thm4qtVox5tr-jIBsghRKgVUT9he4WDqZhIS_o3SYNm5lRZU_L7w4skfN83IGp8zJg95OmVewXd7h03Q-tgm2xwSJoh2wAJgngVSBiPdmrpMWKm_1RCHRwJVBM3crqGleF_u4jyjdWZMdLqFQZOZDZ58rnnq61HOWipMBfROt-RGh_TalpvFCBVA7St2eGwoQB1W-2rd884uzweEddD6G79FxA1JEMmhn50uaygmd2goSA2Ybq2TPUgWB7QLeKkhkZ3L56nAtpfIvcd2Z7TEag4R-XIumiF3TlGAhD-eBBDovXgT7hPaFxcvd4zkPbP0z97ck_aLCDHPNfVH7l11qP8D83NIMc1a-d71YJE74RvcHTiSdmz7jWr8QlSW7GvDBlFa0Zk3I0WmYKwwINveTjnwnXXb9T6bT1SJFuq_QAyJaWf-_FdI0x_MFTvTB_3xFIKp5204EDT5OSaQ7-cDwkJ8=w524-h929-no?authuser=0


After all of the installation and the mission of pressing on bearings etc.  I set out to bleed to brakes. I had made a major mistake and it seemed that all the brake fluid had drained out the rear lines due to a poorly blocked brake line. So I know I was going to have some teething issues.

I started bleeding and surprisingly not too many bubbles came out of the system. However I just couldn't get the pedal to be hard, I probably did 3 bottles of fluid and I bled every joint on the brake system, master cylinder and abs unit included.

So after a week of puzzling over what had gone wrong, I realised that the Megan sport has a slightly larger piston diameter on the brake calliper. 

So this started my theory that maybe my master cylinder is too small. It would take about 3 full pedals before any braking would start. In order to try and prove this theory, I blocked off the rear left calliper and went for a drive, and to my surprisement, the brakes were pretty good, it was soft for about 50% of the stoke and then it was pretty responsive.

Do you think this is a fair test to come up with the assumption that I need to upgrade the Master brake cylinder?

1. On that topic, does anyone know of any suitable master brake cylinder that would be bigger?  I am also worried about this because a bigger MC means that my pedal will have less mechanical advance on the brakes.
2. Can one then also upgrade the vacuum booster as well to compensate for this?
3. Is the stock jimny a dual piston MC or a single-piston MC?

Sort of feeling a bit stuck here with my project, I have spent quite a lot of time and money on this project and I would like to get the car back on the road, Any help and or suggestions would be greatly appreciated. 

Hello I did not do this conversion but I would think:
1) The size of the master cylinder should not matter at all for your issue. The breaking system is a closed hydraulic system and there should be no compression at all. Therefore a small movement of the master cylinder should increase the pressure in the system forcing to break cylinders to move. The ratio of the piston diameters define the ratio of the forces so if you increase the diameter of the master cylinder, the with the same force on the pedal you get less force onto the breaking pads.

2) From your description, I would still think that you have air in the breaking system. To get the air out may be tricky and there are 3 ways in addition to the usual do it yourself style: use a pressurized bottle (maybe 5-20psi) at the master cylinder and open the bleed points. With this overpressure the break fluid flow is higher and will flush out some air, that usually might stay at some high points. This is what I prefer. The second way is with a vacuum at bled points and the third way is tho fill the system from the bleed points, backwards.
 
3) But actually, my recommendation would be that you go to a good repair shop, let them bleed the system and also check if the wheels breake properly with a roller break tester.

4) Did you check your „new“ break calipers, do they work properly? Are they tight? Did you make a caliper service before you installed them?

Good luck and I would take any risks with the breaks.

Increasing the size of the master cylinder will reduce your braking power on the front discs, and the larger pistons on the back will upset your brake bias. That is not ideal!
The symptoms you are describing sound like an issue with the caliper alignment on the disc, such that the pads have to travel a greater distance before the disc is firmly clamped. The same thing can happen when a front wheel bearing goes and the movement in the hub allows the disc to push the piston back in. 

Thank you for this,

I think you might actually be onto something here Because I also noticed that there was a bit of rust on the rear disc, and it was not wearing off even after a decent amount of driving.

I removed the one rear brake calliper, so driving around with one 1 rear. And It now brakes in 1 pedal press.

But something is still off.

Also on this topic,

Say its not aligned,

Surely the first pump would push the piston out?

Then it will sit there, and the second pump would brake properly.

Or does something retract the piston back in after every brake pump?

So my theory is that the newer callipers' volume is larger than that of the master cylinder, hence me needing to pump 3 times before they start braking.

But as you said this is a deep hole I'm diving into.

I don't believe playing with a bigger master cylinder is correct