New Brake Master Cylinder Needed??, diagnosis inside Options

[BonzaiCelica]

I replaced my brake master cylinder with a new Raybesto unit as well as a new proportioning valve (OEM). My problem is I will get air pockets 5 min into driving, pedal sinks momentarily and then brake feel is back to normal

While bleeding brakes, once i crack a caliper open, air bubbles do not come out till about 10 seconds later.

-Booster Operating check OK
-Booster Air Tightness check – engine off OK
-Booster Air Tightness underload OK
-No Leaks at Banjo Bolts
-Soft Lines OK
-No rust between shims and pads
-Caliper Pistons Seals not torn or leaking
-Caliper Pistons move freely when pressed inward with bleeder valve open

[BonzaiCelica]

the only caliper i can't get rid of air from, is the Right Front Caliper. all others are fine.

This post has been edited by BonzaiCelica: Apr 13, 2020 - 4:45 PM

[slavie]

Can you push pistons back in with bleeder closed? Roughly same amount of effort on both L and R sides? The fact that you can push the pistons in with bleeder open is great, means that the pistons are not stuck, but tells you nothing about the condition of the rubber lines, which I would suspect next. Soft lines can look fine on the outside but have inner lining swollen/collapsed, not allowing easy bleeding and potentially causing the caliper to drag. Pressurized fluid can push through with booster working, but not so much when bleeding brakes.

May as well go with SS lines. I think StopTech ones may be no longer available (I had a hard time finding a set in stock), but you can but the Toyota adapters and universal SS line of appropriate length and basically build your own a la carte for less than the cost of StopTech lines.

[BonzaiCelica]

never said anything about sticking brake pads.

i swapped calipers left to right and right to left. the right caliper is still having the issue where it bleeds air pockets no matter how many times i bleed it. ordering one now!!!

[Bitter]

never said anything about sticking brake pads.

i swapped calipers left to right and right to left. the right caliper is still having the issue where it bleeds air pockets no matter how many times i bleed it. ordering one now!!!

You can't swap calipers side to side, that would put the bleeder port below the fluid inlet making it impossible to bleed out all of the air. I actually see this once or twice a year at work and after you've been burned by it once yourself you'll never forget.

Did you bench bleed the master cylinder before installing it? Did you bleed in stages after it was installed? By stages I mean first you bleed at the master cylinder and it's two lines by cracking one open, pedal down, close line fitting, repeat repeatedly until no bubbles, then do the other line fitting the same. Then bleed at the ABS pump inlets and outlets if it has ABS (you mentioned a prop valve so probably non ABS), you can bleed at the prop valve inlet and outlets just the same though, THEN bleed at each wheel. You may need to do 'surge bleeding' where you pump the pedal 3 times and hold it down hard then someone opens the bleeder, sometimes that pulsing pressure motion breaks air loose from where it's stuck. Also sometimes you'll need to bleed with power assist using the surge bleed method to get enough fluid velocity to push little bits of trapped air out.

Finally, if you're using a 'one man' brake bleeder and not doing a final 2 person bleed then that's probably the source of the issue. The one man bleeders work about 90% of the time to get about 90% of the air out. Vacuum bleeding is great to get the process started and mostly done, but it almost never gets all the air. Pressure bleeders ($$$) are closer to 100% but expensive and wasteful, meant for high volume production and big shops, but pretty effective because they can get high fluid velocity. However I would always suggest following up with a manual bleed just to make sure.