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

I think there may be a problem with the way the brakes are being bled. Beg, borrow or steal 3 brake hose clamps. If that is a no go use adjustable clamps ( mole grips ) but protect the hose from damage. Once fitted, push on the brake pedal, there should be little movement and the pedal should be solid. If it is, remove each front clamp in turn and retry the pedal each time. There should be a little more movement each time but the pedal should feel firm. If your findings are as above the problem is in the rear brakes. If when you clamp of all the hoses the pedal is soft and moves appreciably, then the bleeding was wrong or the master cylinder is at fault.

Note that if the fluid had water in it, the master cylinder walls can rust over time and as bleeding the brakes uses the full stroke of the master cylinder piston, the main seal on the piston can be damaged and you will require a new master cylinder. I suggest you carry out the process as detailed above and report back with your findings.

Roger

wrote:

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?

The movement of the caliper with each stroke of the brake pedal is small- around 1-2mm. If the caliper is at an different angle to the disc then they will always get pushed back to the point where they barely make contact. When you start braking an edge of the pad will contact first, making very little braking effort. You have to pump the pedal more times to twist the caliper around and get more pad contact, but every time you release the pedal some of that work is undone.

I'd definitely recommend getting an experienced mechanic to have a look, especially if you are driving around with defective brakes...

So Update to all.

I believe the rear pads are seated properly, when I manually toggle the handbrake lever, I can barely move it, the pads are right against the rotor.  They don't even move 0.5mm.  So that is not the issue.

I went to a brake shop today, to be honest, everyone has been very hesitant to help and or assist me. Went to about 3 places today and eventually, the one guy put the car on the brake dyno, he said it scored perfectly If I didn't say it was modified he would pass it, except for the fact that we have to double pedal each time. (which he says is a symptom of not enough fluid going to the brakes).  

So after a very long rabbit hole of youtube and reading, I have decided that it is the stock combination valve that is limiting fluid to the rear.


This video leads me to this idea.  It seems this combination valve is made for rear drum brake vehicles.  I'm not sure if anyone on this forum has changed this out or bypassed it.  But I see some people install rear brake proportion valves. Im not sure where and if they then replace the stock Jimny one.

But this lead me to an experiment, I went and bought some hose, and I wired the master cylinder directly into the abs module, essentially bypassing the stock proportion valve.  When I took it for a drive, not much seemed to change, however the brakes felt a lot sharper and more aggressive when braking hard, but it still took 2 or 3 pedals to build up some brake pressure.  

So this now has me asking two questions:

1: is the Jimmy brake system got a front-rear split?  so each master cylinder piston controls a different circuit. ( a lot of cars with brake lines to each wheel, will do this diagonally)

The way I have it wired now in my mind should be sending fluid 50:50 to the front and rear


Question 2:
The Jimny's front rear callipers are roughly 51mm big stock. and the master cylinder seems to fill them up.  Why would it not fill up my rear 38mm callipers. I have essentially wired one master cylinder piston to the front system and one to the rear.  Unless the abs unit is also mixing and matching lines ( I doubt it).



 

The basic start to your dilemma is the master cylinder. You must carry out the test that I have mentioned in my previous memo so that we can be assured that the heart of the system is working correctly.

Roger

The 'double press' is concerning me. See this article and diagram how hydraulic brakes work

en.wikipedia.org/wiki/Hydraulic_brake

Essentially it is a sealed system, so when you press the brake pedal the master cylinder pushes fluid to the calipers and pushes the pads on to the discs. When you release the pedal the master cylinder sucks the fluid back from the calipers. If there is too little or too much fluid, this is topped up or removed to the reservoir.

The fact that you can double press the pedal and get more brake pressure sounds like either you have air in the system or there is a leak, possibly in the master cylinder. If there is no air or leak then the pads wouldn't move any further on to the disc as when you release the pedal they move back by the same amount, not hold where they are and move in again.

When you 'double press' is it okay with a single press for a few seconds afterwards or does it always need a double press?

Either way, I would be surprised if it would pass a MOT in the UK.

Robin