Brake system funk - open to suggestions
 

I bleed brakes, they're solid as a rock. For the next few days it's great, can run an event on them, no issues. Following the bleed, about a week later the pedal feel begins to get softer. Two weeks out, pedal has tons of travel and feel is very mushy.

I've bled several times to make sure I'm not just pulling a few more trapped bubbles out, same slow degradation each time.

My best guess is there is some very minute leak somewhere that allows pressure loss over time. But, I can't for the life of me find any signs of leakage.

I'd love to troubleshoot/narrow down the possible causes some way that doesn't include just throwing money at things until I finally replace the part that had the problem..
Open to suggestions about what element may be causing the issue, and/or how to possibly pinpoint the cause.

Specific setup is Wilwood dynalite fronts, rx-7 rears, braided lines all around, reservoir and booster were just replaced recently with 'fresher' used units since mine had taken a beating from turbo heat. Perhaps one of those is faulty..

Thanks for input :)

-Ryan

I had a rear braided line with a poorly machined junction block that leaked. However, that leak was obvious.

I think you're on the right track to be suspicious of the parts that are different from before. Do you still have the master cylinder you swapped out? You could put it back in for a test.

I think I tossed the old master, but I believe it was beginning to have this issue before I put the new master and booster in, just not to this severe of a degree..

I have a friend's car taking up the garage, making a 3" exhaust for it. I should have that out by end of today and I think I'm just going to pull the car in, put it on jack stands, pull all the wheels off and go over every inch of the system with a flashlight and see if I've missed something.

Another thought is perhaps a piston seal in one of the calipers could have a small weep? If it's a front, I'm getting the brakes up to temp, and it's a small enough leak, maybe it burns it off so it never creates a fluid trail..?

-Ryan

I wouldn't be worried about fluid getting out, but rather "how is air getting in?"

If you had a tiny leak which takes days/weeks to fully develop into a noticeable problem, the symptoms would be an empty fluid reservoir. Until you emptied that reservoir, you wouldn't notice a difference.

I can't see how you would have water in the system with as often as you've bled it, but if perhaps you did, (DOT5 Brake fluid does not mix with water, the water will sink to the lowest point in the brake caliper) the heat generated by braking could cause boiling of the small amount of water in there which would lead to bubbles.

Can you confirm which type of fluid that you're using, and can you also confirm that you're putting in fresh fluid every time?

RBF-600, I never recycle fluid, always date the bottle after opening and never use anything over a 4-6 month shelf life..

Yeah, water in the system seems unlikely.. not sure how that would get in there. The dynalites have bleeders top and bottom, usually you only bleed the upwards-facing ones, but I could do the bottom ones, which could have a chance of pulling out water if there was any..

I agree, "how is air getting in" is basically the key question. I suppose I was thinking of fluid out/air in as being mutually inclusive: if air was getting in, fluid would be coming out of that point...

My welder is acting up, so I've got an almost finished exhaust on my bench and a car still taking up the garage, so at least another day before I can pull mine in and do a thorough check of everything. Balls.

-Ryan

I would think that if the leak in the system were large enough for air to flow in, then the brake pedal would drop to the floor even after bleeding because your loss of brake fluid would be so substantial. I would expect the sequence of "fluid getting out" to work like this:

1. You push brake pedal, and extreme pressure forces a small amount of brake fluid through a breach in the system for the entire duration of the braking sequence. (We're talking somewhere between 500-1500 psi here)

2. You release brake pedal and (if we were in the world of a "perfect vacuum", which we are not) your lines see -14 psi of pressure for a brief fraction of a second. Roughly 1/30th to 1/100th the pressure of "on brakes" over a much shorter period of time.

3. As the master cylinder releases the pressure on the brakes, extra brake fluid from your reservoir flows into your brake system.

Because of the massive pressure differential (brake fluid has 500-1500psi of motivation to escape, but air can only have a maximum of 14psi of motivation to enter) I would expect you to see massive amounts of brake fluid loss before you saw any amount of air entering the system.

Edit: I recall at one point in time I had noticed air in the master cylinder of my motorcycle. I was apparently losing a tiny amount of brake fluid. Over a period of about 4 months, the level had fallen from just below the cap and seal to just above the bottom of the reservoir. On a motorcycle, the reservoir stares you in the face every time you ride, so it's easy to pay attention to your brake fluid level - unless you're a moron and you have, for some ungodly reason, covered your brake cylinder with some fluffy sock that says "orange county sluts" on it.

Anyways, back to the story. I stopped at the auto parts store to get some fluid because it was starting to appear dangerously low. Upon opening the cap and removing the big fat rubbery seal underneath, I discovered that my brake reservoir was not empty...it was instead full of a clear substance with a remarkable resemblance to water??? That one caused me to do a complete flush of my brake system.

Once the pedal has become mushy, are there any bubbles present when you first bleed the calipers? Is the fluid still clear or is is darker? Is it milky?

What does the heat shielding look like for the front left brake line where it passes between the exhaust and the frame rail? And the proportioning valve/other lines near the master cylinder? Pics please?

Sounds like brake master to me. I'd check seals on the calipers as well. I used to have a dynalite caliper system on my old mini and that particular setup was very sensitive that both top and bottom bleeders had to be bled.

Although that doesn't sound like your problem as it goes from good... to no bueno. :vash2:

I had a similar issue a couple years back. Turned out there was tiny leak in the rear caliper adjuster mech. That channel was always wet (a few drops of fluid) when I got in there on pad change. I swapped out the caliper and everything was perfect again.

Ryan,

BTW I keep a bottle of this in my track emergency kit. This stuff is pretty good at sealing up brake line fittings when you need it. I used to use it on my motorcycles back in the day because I had speed bleeders installed... now I just keep it around because it's saved my bacon so many times.

Speed Bleeder Thread Sealant | eBay

Sounds like the master is leaking into the booster. Separate the master from the booster and check for signs of wetness.

My money is on weak master.


Does the pedal drop at all over time after the pedal gets spongy?


If you're not loosing any fluid, you have basically only the master to look at.

I usually bleed with a clear bottle but opaque line, going to use a clear line this time, will report back on this.

Looks like Wednesday will be the day I'll be able to look into the brake system.

I don't expect that heat is affecting the system, new heat shield system fully protects all the brake lines, master, and prop valve except for a 2" section of that front left line:

Bottom section w/ ducting that 'blows' air over the turbo system (this is evacuated out of the hood):

https://www.miataturbo.net/attachmen...ine=1404807694

With top section in place:

https://www.miataturbo.net/attachmen...ine=1404807694

Thanks for the input Erik, Wilwood told me you disregard the downward facing bleeders, they are just there because mounted on the other side of the car, they would be the upwards facing ones. Next time I bleed the brakes though, I'll do the bottoms before the tops and see if that helps any.

Thanks for the suggestion, I'll look into the master. Yes, pedal develops more movement before brakes begin to engage over time along with becoming more spongy. I can separate the master from the booster without draining any fluid right?

Thanks for the link, If I find any fittings that appear to be leaking I will go for that fix.

btw, you can hear the bubbles when you are bleeding. sounds like a lot of little crackles. Clear line helps though.

You can split a piece of fuel hose lengthwise and it will go right over that small section of brake line to insulate it. That would work with what you have already. Everything helps.

Dont bother with the bottom holes. There cant be water trapped there. The reason your brake fluid absorbs water is so such a thing is not possible.

^^ My money is on this also. Fairly common problem, telltale signs are a dribble of melted paint under the master down the front of the booster.

Got the car in the air and went over everything closely.

Master is not showing any signs of a leak. Disconnected it from the booster and checked there, everything is dry.

All lines appear OK.

On the front dynalite calipers - on both the left and right caliper- the top/inboard bleed nipples have a bit of what looks like oxidization around them. Looks like there was fluid there, but not a lot, and this may just be from spillage when bleeding them - I always spill a little fluid when removing the bleeder hose...

https://www.miataturbo.net/attachmen...ine=1404958335

The other thing that is a bit quizzical is in the rear. These are RX-7 Turbo FC calipers, first of all there are two bleeders. I had forgotten about this - the downard facing one (pointing left in this pic) is almost impossible to see when it is mounted - I have only been bleeding the higher of the two, but some googling shows that some recommend bleeding both.

Secondly, this pic is of the right rear, and the boot/seal around the piston is missing. It is present on the other left side. I think there is an inner and outer seal, and it doesn't look like there has been any fluid escaping from here, but perhaps the absent outer seal is causing an issue...

https://www.miataturbo.net/attachmen...ine=1404958335

I cleaned up around the bleeders on the front calipers with a wire brush so that if anything else develops I'll see it and I bolted everything back together so I can do a bleed that includes the lower bleed screws as well to see if that does anything...

Personally I would always suggest bleeding with a clear line. This allows you to see what is coming out of your brake lines. It is important to watch for bubbles but also to keep an eye on the color of the fluid.