Wilwood Master Cylinder size

[Otee453]

Hi friends,

I’m stumped. Manual brakes, Wilwood pedal box, ¾” master cylinders each for front and rear brake circuits.

I’ve bled these things 47,000 times, including bleeding the masters 3 or 4 times (connected line to pressure outlet, routed back into reservoir). I am confident that I have no air in the system. I put speed bleeders on all calipers (these things are a great invention), so I’m not getting air in at the bleed screw. I’ve even built a pressure bleeder and pushed fluid through the system a couple times. I am bleeding the system according to Wilwoods instructions, front & rear simultaneous, right, then left side. I’ve played with the balance bar (in-out, 2 quarters of space between clevis, etc...)

When pressing the brake pedal, I get travel all the way to the floor and very little braking during go-cart. I have the FFR dual piston calipers up front and single piston calipers for a 95 Cobra Mustang on rear.

When I push the pedal and watch the balance bar, it appears the master cylinder piston on the front circuit is traveling just about an inch and stopping while the rear circuit is traveling in quite a bit further, almost like there is no resistance or pressure building in the master. The balance bar thus does not allow any real force or pressure against the front piston as the rear just continues to push in. I have put a cap on both master cylinder pressure outlets and they are SOLID when pressing the pedal, so the master isn’t bad.
Also... NO LEAKS IN PLUMBING ANYWHERE. Sahara dry at all unions, banjo bolts & caliper pistons.

I called Wilwood. They recommended going to a ⅞” master for the rear circuit. Before I drop ~$100 for a part swap experiment.... any thoughts? I’ve researched brake volume calculations and a single piston calipers should require less volume than the dual piston in front (unless my cursory education on fluid dynamics is wrong)

Thanks
(Just to be clear, I’ve read multiple brake problems posts and the problem is generally resolved by bench bleeding the master cylinder. I’ve done that a few times and get absolutely ZERO bubbles, likewise at the calipers)

[totem]

It’s unlikely to be the diameter of the master.

The pedal is supposed to be rock hard.

If you are confident in the bleeding, disassemble the rear brakes to ensure that nothing is twisted, stuck or binding. Pads, sliding pins, clips, etc


I once had a pad with one corner stuck in a wear groove on the caliper mount. The pad was flexing and springing back, creating a soft pedal.

[Otee453]

It’s unlikely to be the diameter of the master.

The pedal is supposed to be rock hard.

If you are confident in the bleeding, disassemble the rear brakes to ensure that nothing is twisted, stuck or binding. Pads, sliding pins, clips, etc


I once had a pad with one corner stuck in a wear groove on the caliper mount. The pad was flexing and springing back, creating a soft pedal.

Good idea. I’ll do that

[rich grsc]

The suggestion of a larger MC for the rear is wrong. The same size or smaller for the back brakes on these cars

[Otee453]

The suggestion of a larger MC for the rear is wrong. The same size or smaller for the back brakes on these cars

Thank you. That confirms what I was thinking. I will chase other things and not buy a larger master.

I questioned the tech guy at Wilwood on why FFR (he was very familiar with FFR) would supply something less than adequate. I even asked how a small single piston caliper would require a larger volume than a larger dual piston caliper would. I explained my cursory knowledge would be that a single piston caliper would require a lower volume but higher pressure, hence a smaller master, not bigger. Apparently the tech guys at wilwood are programmed to say “go bigger” when nothing else explains anything.

I have one other thing to check... on my rear circuit I made one tight 90 degree bend in the line. I didn’t think it was too tight a radius but perhaps it narrowed the flow enough to limit the volume to the caliper.

[jrcuz]

Wilwood has a great video on their website on how to bleed their brakes.
JR

[CraigS]

Check to be sure the MCs are returning fully when your foot is not on the pedal. Some times the pedal can hit a frame tube that reduces how far up it can travel. If it is hitting, you may need to adjust the MC pushrods. It's probably best to pop the rubber bellows off the MCs to assure the pistons are coming all the way out to the snap ring.

[Otee453]

Check to be sure the MCs are returning fully when your foot is not on the pedal. Some times the pedal can hit a frame tube that reduces how far up it can travel. If it is hitting, you may need to adjust the MC pushrods. It's probably best to pop the rubber bellows off the MCs to assure the pistons are coming all the way out to the snap ring.

I’ve checked to be sure the pedal isn’t hitting the frame bar (it’s not), it also feels as though the pistons are fully retracting. I will pull the boots back so I can really see if they are fully retracting.

I’ve now also pulled the calipers off and removed pads. The pistons extend when I pump the brake pedal and do not seem to retract on their own. Calipers seem to be functional, so I “screwed” the pistons back in, reinstalled everything and still have too much pedal travel and sponginess on the rear circuit. Interestingly, when I push the pedal all the way down and hold it and quickly release and reapply the brake, the pedal doesn’t travel as far and feels more solid. I keep thinking keep its air in the circuit but I’ve bench bled master and bled at the calipers multiple times with no bubbles.

I’ve ordered a brake pressure gauge with fittings, so I can do some diagnostics that way. I’m stumped. I’ve bled brakes on all sorts of vehicles before from cars & trucks to dirt bikes, atv’s & sXs’s. Heck, even bled brakes on my boat trailer with surge brakes.

[rich grsc]

Have you adjusted the parking brake? the PB needs to be cycled so there is no excess play in the rear calipers. If there is slack in the PB mechanism, the pistons have too far to move and build pressure.

[Otee453]

Wanted to update for those doing future research.

I had done everything suggested in this thread and my rear brake circuit actually continued to get worse... worse as in too much piston travel. After purchasing a brake pressure gauge, I was getting 0 pressure at the rear calipers (yes, ZERO). At one point I couldn’t even push brake fluid out of the caliper bleed screws with the brake pedal.
I became convinced I had a bad master cylinder, or due to my “plumbing”, I indeed did need a larger master despite the math. I also began wondering if the rubber brake hoses (each caliper & center hose) were expanding and I needed to go to braided stainless.

I disconnected some of my hard 3/16 brake line and tested pressure at various points. Of course after opening the brake lines I needed to re-bleed each time (both master and lines/calipers). I also removed the rear master, disassembled & reassembled it just to see if they was any apparent problem (none). I then started wondering if my reservoir hoses were routed just a bit higher than my reservoirs. Indeed, it appeared that at the highest point of my reservoir hoses going to each master may be as high or slightly higher than the top of the fluid level in the reservoir, particularly if the reservoir was only half full. I shortened the reservoir hoses slightly so they would not route as high inside the foot box area.

After pressure bleeding a full quart of new fluid through the rear circuit.... it appears I am in working condition. I think the routing of my reservoir hoses was just a scoche too high compared to my reservoirs. If I have my reservoirs full to the top, the fluid is above the highest point in the system. I hope there is no brake fluid expansion from heat