so i put a NEW master cylinder, braided lines and rebuilt 4 piston calipers into my 91 fc, trying to fix a mushy brake pedal. we've bled the snot out of it, and the pedal feel is now worse!



WTF?



my t2 was the same way. the 1st gens always feel good, i dont get it

Maybe a weak booster?

if anything its too strong, i dont know what else there could be though?

did you bench bleed the master cylinder? Check the bleeders for stripping where they install into the calipers. I had that problem once, and I needed to get a new caliper because the bleeder stripped.



Also, when you bled the system, did you press the pedal slowly or quickly? quick pedal pressing will give you more, but smaller, air bubbles, which would give you a mushy pedal.

we pedal bled it twice, with different people, and then we pressure bled it.



dont think its a stripped bleeder, its not leaking fluid

I'd look to the master cylinder. If it wasn't bench bled before installing, it could be your culprit. Any air in the master cylinder is a bad thing, and it's very highly reccomended to bench bleed it before installing specifically to prevent the spongy pedal feel. Even pedal bleeding/pressure bleeding with everything installed can take many cycles to get everything to where it should be if the MC wasn't pre-bled. You would know it if you bench bled it.





From SuperChevy.com:



That being said, if it was bench-bled, I'll have to think harder!

would a missing/failed check valve at the booster cause this?

how is the pedal with the engine not running? if it is spongy without vacuum assist then there is a hydraulic problem. and he is right you should ALWAYS bench bleed master cylinders before installation

Bench bleed it. If you can't bench bleed it. Then take two old hard lines. Cut them and keep the flare and the nut. Put a plastic hose at the end of the cut hardpipe and zip tie them on. Fish the lines back into the reservoir and pump away. I did that when I installed my new master.



I just installed the FD calipers I bought off of you a long time back. New seals, new pistons, new boots, new mazda 929 master. new rebuilt rear calipers. SS lines. THe pedal is rock solid.

If all else is good the only thing I can think of is that you bleed it to fast. If you go pumping the **** out of it you will never get all the air out of the lines. Nice and slow pumping does the trick the best.

My old brakes used to feel really mushy before I put the FD calipers on. Now that I have the 929 master and the Wilwoods, you barely touch the brake pedal and your bouncing your head off the steering wheel.



I had a sneaking suspicion that my old calipers were sticking or not operating properly. But then again, they had 200k miles on them

Couldn't really soft old brake hoses contribute to a mushy pedal?



Never mind, I see you installed new braided lines. https://www.nopistons.com/forums/pub...1047683664.gif

i looked in the FSM (imagine that https://www.nopistons.com/forums/pub...IR ) and it says mushy pedal can be



push rod between the master and the booster

air in the lines

worn pads

air in the lines

pedal free play

and air in the lines (it does say air in the system more than once!)



so i think i get to pull it apart and readjust the pushrod in the booster, yay for me

i think the booster is bad, which is a new one, but the car has been sitting for like 10years, in a field....

I have a similarly vague problem. My brakes work great, eventually. I can feel brake pressure being applied the first inch or so of pedal travel, then it gets real firm, and stops great. I would adjust the pedal so it was immediately firm, but then the brakes would be dragging. What's up with that? Squishy rubber lines perhaps?

yeah, or old fluid?

i adjsted the pushrod on mine, it helped a bunch, but its still weird. i adjusted the pushrod between the booster and the master

hey cymfc3s, how's the 929 master working for you? I'm interested in doing that too, but not sure what parts are needed...did you need a new booster? Please let me know what other parts are necessary, thanks!

you need an s5 t2 brake booster, or fd. the 929 master wont bolt to the s4 and na s5 boosters.



good news though it does look like the rx8, fd and fc (an 6..) all share the same brake booster to car bolt pattern... so upgrading to an rx8 master and booster might be a bolt in. the rx8 master and booster are like $100 each, new

That's interesting. But it's not really an upgrade. Could be a downgrade if it messes up the stroke/pressure/assist etc.....



I wonder what size bore the rx8 master cylinder is. The 929 is a 1" bore. The S5TII ones are 15/16" bore. I like the 929 master a lot. Full brakes is like 1.5" travel. Then it's rock hard and you get to modulate the brakes based on the pressure/effort you apply on the pedal rather than trying to move it 1 mm more or less to modulate.

i know the tribute is a 1" master too, i dont know the sizing on everything else, but an s5 t2 booster is not exactly easy to find..... but there are a lot of mazdas that share the same booster to firewall bolt pattern, and the rx8 stuff is CHEAP. the s5 t2 booster is like $300 and the rx8 is like $90

so is it fixed now? what was the solution?

nope, its a little better, but still shitty.

Maybe the master is pulling back fluid?

pulling back? https://www.nopistons.com/forums/pub...IR#>/blink.png

Bench bleed

we were just talking about this today, both my verts have had mushy breaks, i replaced all calipers, ss lines, slotted rotors and pads still the same.



Dunno

You dont follow directions. Bench bleed the master, bleed from the furthest point to the closests point. Rock solid brakes. Simple.

did that twice! 2 different people! 2 seperate occasions!

does your car have a proportioning valve?? my 88'vert has one right next to the master. it is the only part i can think of that has not been covered already in this topic. this is very important in distributing pressure between the front and rear.

Yah, like when your letting go of the pedal, fluid is getting drawn back. Pistons moving freely in freshly honed calipers, now with more clearance. Just guessing at this point.



Im not gonna tell you to bleed your brakes, im fairly confident you know how to do that.

might be? i dunno.



i bled it once, jack (who is a master tech) bled it once. we prolly put about a gallon of fluid thru it.

you are telling me those clowns of master techs at mazda can't figure it out?

his car works....