Brake Booster Problems with Heat

[bingo]

Hi all, I searched for this and couldn't find anything. I have a 72 240z that has all new brakes except for the booster and vacuum check valve. They have been working great, and still work great under normal use. The problem I am running into now is after tracking it for a few laps the booster starts failing, thankfully every time it has happened it was coming back from the pits so I knew it would be an issue and could take the track appropriately slow so it wasn't scary. If I pump prior pedal pressure is normal, if not I get the booster thud (no pedal pressure until the pedal is like 1" from the floor). As you may have seen in one of my other posts I made my own brake booster lines which seem to be working perfect, but I made those thinking that maybe the old ones were behaving odd when hot. The new ones don't seem to give enough even when hot to cause the problem, but I am open to any ideas. I don't think it is brake fade, after pumping 1x the brakes feel totally normal again, not squishy like with fade.

Any ideas on how to diagnose this? I can't really replicate unless I am on the track and ideally this will be fixed prior to going to the track next time. Is it time for a new booster? A new check valve? Other thoughts?

[Patcon]

I wouldn't think the booster would be affected by tracking the car.

Maybe one of the track guys will chime in

[Zed Head]

35 minutes ago, bingo said:

 If I pump prior pedal pressure is normal, if not I get the booster thud (no pedal pressure until the pedal is like 1" from the floor). 

You said that you made your own "booster lines".  When you say booster do you really mean master cylinder?  The booster just has a vacuum hose.

Pedal going to 1" from the floor would not be a booster problem.  The booster just adds assistance to the mechanical parts.  But there is a solid metal path (with one chunk of rubber in the middle) from the pedal to the MC.  Sounds more like an MC seal problem. 

[bingo]

I should have said booster hoses.

This makes sense, there is a direct connection so going way down wouldn't be the booster and I couldn't figure out how the booster would be impacted by heat. The master cylinder seems more susceptible, but it still seems a bit odd. I did order a new one and hopefully that fixes it, it is time for the annual flush anyways. Has anybody else had heat related problems with the MC?

[Patcon]

12 hours ago, bingo said:

I should have said booster hoses.

This makes sense, there is a direct connection so going way down wouldn't be the booster and I couldn't figure out how the booster would be impacted by heat. The master cylinder seems more susceptible, but it still seems a bit odd. I did order a new one and hopefully that fixes it, it is time for the annual flush anyways. Has anybody else had heat related problems with the MC?

I suspect you are boiling your brake fluid or the water in the system. You might want to consider converting to a better high heat fluid if you are going to spend any sort of time on the track

[Zed Head]

35 minutes ago, Patcon said:

I suspect you are boiling your brake fluid or the water in the system. 

I was thinking the same thing but got distracted when he said it wasn't like brake fade.  Boiling and brake fade are different things.  Boiling would kill the pressure immediately.  I was going to suggest that a brake line might be close to an exhaust pipe or engine part, which might explain why it comes on so soon.

Check all of the lines, shield the MC from heat, make sure that the brake fluid is pure and fresh.

[Patcon]

Lots of guys who track their cars actually bleed their brakes between sessions. Most brake fluid is hygroscopic (absorbs water from the air) and it does so even better when it is hot. By bleeding the brakes, your get fresh dry fluid and remove a lot of heat with the older hot fluid

[bingo]

So boiling fluid seems possible, but would it go away by pumping 2x? When it starts happening I push brakes, and get almost no braking but if I let off and push again they seem pretty normal. Fluid doesn't seem crazy hot, and I don't see boiling when I open the reservoirs. 

I will replace fluid, and check the brake lines for heat, there is one that is by the headers so I can insulate that, and I still might just go ahead and replace the master cylinder since I already ordered one, but I guess I could replace fluid, insulate and test and then it can be a good learning experience. 

[Zed Head]

A hot spot, boiling, creates a vapor pocket.  Just like air in the system.  Pumping compresses the vapor pocket, just like it compresses an air bubble.  Seems plausible.

[jonathanrussell]

This is what I use to flush  the system before track days...not my Z my MX5. If / when I ever do a track day with my Z I will do the same. With this brake fluid I flush at least once per year. If I do more than one track day per summer, I bleed and move a decent amount of fluid through the system for each day.

Castrol SRF

 

[bingo]

I think it was just moisture, but a track day in a little over a week will confirm. The car had been sitting for 17 years prior to the last fluid change, so I am guessing the new fluid absorbed a ton of moisture from the new brake parts and all of the old lines. I put in new dot4, will drive around with it for a few weeks and then switch to some dot4 racing fluid.

I also made a heat shield since the header was only like 3 inches from the brake junction (I am not 100% sure what it is called), but the question below with a pic is the part in question. Heat shield pic is here, it is hard to see because it is polycarbonate so you can mainly see the shiny surface that reflects the other stuff on the frame rail. Top of the pic is the front of the car. The heat shield has 1500 degree radiant heat barrier on the other side close to the header, like what you can see on the top of the carb heat shield directly above this heat shield. It seems to be effective, on spirited drives that area is basically the same temp as the rest of the engine compartment, not the same temp as right by the headers. 

 

Before I bleed again I wanted to see if you knew what this thing is, mainly I am wondering about the piece in the middle of the junction block that looks like it maybe should be a bleeder valve, but doesn't seem to have flats to bleed with. Should this be there? Should it be a bleed port instead of this? Currently it seems like a place to capture air bubbles.

 

[Mike]

33 minutes ago, bingo said:

Before I bleed again I wanted to see if you knew what this thing is, mainly I am wondering about the piece in the middle of the junction block that looks like it maybe should be a bleeder valve, but doesn't seem to have flats to bleed with. Should this be there? Should it be a bleed port instead of this? Currently it seems like a place to capture air bubbles.

 

@bingo, this is actually a fluid distribution valve with a warning switch.  It's a stock item, and is supposed to be there.  The probe on the top is supposed to connect to a wire that leads to a light inside the cabin.  When the fluid becomes out of balance, the switch will close the circuit and cause the light to turn on in the dash.