Suspected brake master issue

[cbez]

This is a long shot but I had it happened on my 69 SRL clutch hydraulics. The rubber hose to the slave was old and worn. When I first got in the car the clutch pedal was soft and after a few pumps it was firm and held the firm pressure for the drive. The next day, soft again (repeatedly). I wasn't until I tried to bleed the slave that I noted the hose was blown up like a balloon in one 1" area of the hose. Replaced the hose and bled, good to this day. So, check all three of the rubber hoses for damage where the sidewall of the hose is weak and can balloon. Look at them when the pedal is firm

all the soft lines are new stainless ones from sin city, no sign of being compromised anywhere.

[JT68]

Suggest you bleed the master a lot more and then bleed the whole system a lot more. Run at least 2 pints of fluid through the master-SLOWLY.

What type fluid are you using?

If you want your master tested we have a master cylinder test machine. (specifically for this type problem) It is easy to tell on it if there is an internal or external leak of any type.

[cbez]

I will do some more bleeding this weekend. I probably did like 50 pumps on the master today just going from the bleed screw back to the reservoir with some tubing.

Using dot 5.

Bleeding is cheap and easy and I don't mind doing it (have speed bleeders and a little magnetic container with tubing, simple as can be).

My usual experience with air in the system is an overall squishy, crap feeling pedal that firms up as you get bubbles out. Not like I have where it's occasionally totally soft in the morning (literally feels like only the return spring providing any resistance) but then great while driving.

[redroadster]

Did you have dot 5 in the system before new
as dot 5 silicone fluid turns to gel in contact with dot 3 , dot 4 , a minut amount , you cant even clean it out as the old fluid impregnates the rubber o ring causing a gum up plus your brake safety valve holds fluid ot should say on the bottle - only for use in marked dot 5 system's-
I tried on my clutch ( new mc , slave cyl flush out line ) it gummed up in 3 weeks
I saw your video and thats not a classic air in the lines cond.

[cbez]

Yes, it was a totally fresh and dry system and has done about 700+ miles give or take before this issue came up.

[davdav2000]

I believe that dot 5.1 is compatible with dot 3 and dot 4, but dot 5 is NOT. Dot 5 and dot 5.1 are very different, as far as I understand, per what red roadster said above. You cannot mix them.

[redroadster]

Yes, it was a totally fresh and dry system and has done about 700+ miles give or take before this issue came up.

??? on the 5.1 fluid , .. most of the reason for new fluid is a higher boiling point needed or on extreme cases needed , like 1ton with a boat trailer, no reason to use in high summer heat ,not warranted on a lite weight and driven car .but as a general rule completely different chemicals applied to rubber aint good for it , .and I as well thought the new componants would work on my clutch system as I was using new master and slave cyl s cleaned the lines out it worked for 2 months , then gelled in the lines ....the cylinders o rings are lubed with dot 3 brake fluid and a different o-rings used like in a/c systems plus even cleaning the o rings isn't enough its in the rubber , and the safety valve is almost impossible to get all fluid out ,do not try taking that apart.
I ended up replacing a clutch line and hose to fix it.

[JT68]

If silicone dot5, that may be part of the issue. Aside from the compatibility concerns above, I find dot5 silicone fluid harder to get the bubbles out. (In my experience) it seems to form smaller bubbles so you have to pump the pedal much slower to avoid bubble formation in the first place. Bleeding these systems in general takes quite a while-multiple sessions.

Dot5 is great regarding painted surfaces and works fine once it is fully bled. I have not used 5.1.

So this is a brand new master cylinder? (You stated “fresh and dry” system above)

[cbez]

To my knowledge yes they used a NOS master. But that means it sat on a shelf for however long and then dry on the car for 10+ years before I came along to finish it. Which is why I suspected a potential master issue.

[cbez]

https://vimeo.com/650638613

Video, like four floor pumps this morning to get pressure

[Bwk2000]

Is this just shadowing?

FBC17FF9-8F61-41DB-B04D-48EE6DF4622F.jpeg

[unklpat]

I'd probably put in standard dot 3 or 4. I doubt anyone using our stock brake systems for normal driving needs the heat characteristics in synthetic fluid, and bleeding as part of your yearly maintenance takes of the moisture issues. My only experience with dot 5 had a softer pedal. Your issue looks internal to me, not air in the lines. Pat

[cbez]

Is this just shadowing?

FBC17FF9-8F61-41DB-B04D-48EE6DF4622F.jpeg

Yes, that's a little circle that's part of the casting.

I re seated one questionable fitting and cleaned it really well so I could monitor it, plus bled the master some more for kicks.

I am dot 5 gang all the way just for the paint protection alone. I hate glycol brake fluid.

[redroadster]

I bet I know whats going on with it now , as my 68 123gt volvo did this same thing , not dot 5 though
The front MC piston seal is jamming in the cyl coming off the steel rod ball / socket then popping back on and works correctly bet from the gelling up best to tear down
or ,really replace
Put some dot 5 & dot 3 fluid together mix some and heat to 140 f for a test because it cant be cleaned out
Its bad to fully bleed a well used mc as the gunk in the part of the cyl.that the piston never travels is varnished and has alum & steel fragments
at datsun we stopped doing brake fluid flushes too many came back as the mc just failed all of a sudden ,for this reason

[JT68]

Instead of guessing/trying random fixes, we can test the master if you like, and you will know for sure whether the master has an issue or not. If it holds 1000psi for a few days, it has no issue. If it has a seal or cylinder problem it won’t hold 1000psi for long.

There isn’t anything inside the master that I can think of that would cause the problem you are describing other than air.

Think about it: when the pedal is a rest, the seals are doing nothing except keeping fluid inside the cylinder. Nothing is under pressure. The fluid can’t go anywhere except if it leaks out. Gravity is always trying to fill the master cylinder.

When you pump it up, you are pressurizing the system and compressing the trapped air. (Air is highly compressible, brake fluid is not). That pressure is briefly maintained by the master. There is a simple self bleeding residual pressure valve in the master. Every time you use the brake while driving you are pumping it up slightly. (It has to self bleed/cancel or it would lock the calipers). That is why it feels fine while driving.

Once that slight pressure subsides, the pedal is soft.

Some folks have trouble with dot5, but thousands of people use it without issue. It performs perfectly once you get the bubbles out. Bleeding technique is important.