Does anyone know if there would be any benefit to swapping out the 3.5" brake master cylinder for a 6" one from an 06 AWD model onto a 06 G35 Coupe? Also, is there any significant difference between the two aside from the fluid lines being on the opposite side? I'm assuming bigger is better, right?


The main reason I'm looking to do this is so I can install a stillen master cylinder brace to correct for the spongy pedal feel. Also, I've been having problems with my rear pads having excessive wear over my fronts so I'm hoping a new one will help with that too.

 

The only thing you will really change is pedal travel distance. It is actually an inverse characteristic, a smaller bore takes less force (softer brake feel), but will build more pressure at the calipers for higher gripping force. A larger bore takes more force (harder brake feel) but produces less pressure at calipers. A large bore will move for fluid for a shorter pedal travel, but produce less line pressure. It doesnt mean its better or change 'spongy' feel as you describe, or change brake pad wear characteristics. Nissan engineers have spent alot of time picking the correct bore size/ratio for the best balanced brake pressure build-up and brake lever travel. I wouldnt touch it unless you were changing the caliper size for some reason.

The spongy feeling you describe is most likely from air in the lines. Try bleeding your brakes.

 

Ok, ill keep that in mind. since we're on the subject of bleeding the brakes, after I initially bleed them I have a high stiff pedal that I am looking for but once I do a moderate to hard brake the pedal loses its stiffness and becomes soft and kind of spongy again, although usually if I pump them I can get some of the stiffness back, it quickly fades. I have no leaks in my lines as far as I know as my fluid levels haven't dropped since my last bleed. I do have stainless steel lines arriving today to help with the softness but I fear it may be something other than just the rubber lines being the cause. Any thoughts on this?

 

Most likely they were not bled properly and there is still some air in the lines. Make sure bleeder screws are tightly closed and you used proper fluid, no DOT 5. Make sure there is enough in reservoir, so youre not sucking in air. Get a hose on the bleeder fitting. It should be long enough to go into a cup so it can't pull air in. If the hose is immersed in fluid, air should not get back into the system.

 

That's not how master cylinders are designed. They are sized to take into consideration the surface area of all the pistons in the calipers (slave cylinder) as well as pedal arm geometry to provide a certain pedal effort.

MC's are measured in bore diameter. While the RWD model has a short MC, and the AWD model has the longer MC, the bore diameter is more critical. Both cars wear the same brakes. Documentation I've found suggests both MC's offer a 1" bore...so basically the same MC other than physical length

If you went to a larger diameter bore MC, pedal effort would increase, but stroke would shorten. If you went to a smaller diameter MC, pedal stroke would be longer, but pedal would get softer. This is not a way to fix the spongy pedal feel. MC sizing is part of designing the brake system.

 

I fought the spongy pedal issue for a few years. In the end, I discovered the cause was seized caliper pins, and poor brake bleeding.

Ended up replacing both rear caliper with remans, and a good quality bleed with a pressure bleeder. My brakes are no longer spongy at all.

 

Okay so I've been dealing with the spongey brakes also and mostly everyone has told me to get a new master cylinder all together. I have only 70k on my G and I thought it was a little too early for that. So the caliper pins and a good bleeding really worked?

 

Thanks, I'll try that when I get around to doing my brakes next weekend. I just picked up some steel lines and a power bleeder so ill make sure to give my calipers a good cleaning and free up anything that may be stuck while I'm at it.


So for the MC, If I want to swap it for the longer one to fit a stillen brace there wont be any difference due to them having the same bore?

 

For me it did. I basically went to all 4 calipers and found one of my rears with a semi-seized caliper pin. I replaced both rear calipers and pressure bled and the issue was gone.

I chased it for a while, and almost changed out the MC myself thinking one of the internal seals was leaking.