Spongy brake

Arde

Arde

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The all fluid replace project went well but flushing and bleeding the brake fluid brought a softer brake pedal. The symptom actually is really a firm pedal but sometimes it is firm for a second and then softens and firms up again at a later point of travel. If I release and press again it firms up at the original point.

With the engine off the brake pedal is consistently firm.

Bleeding was done with vacuum the first time and manually the second time. No air bubbles came up the second time so not sure there is air there.

Reading the archives I see master cylinder and rear valve as potential culprits.

The problem started with the brake fluid flush, so what is more likely to be the cause? Should I just replace the master cylinder or spend more time diagnosing?
 
i firmly doubt that if you hadn't the problem before you have provoked the problem during the process (i mean breaking master or rear valve)

if your only job has been changing fluid, the problem is the fluid or the changing, aka the bleeding process

bleed the calipers, but bleed the clutch cyls too

i do not know how many fluid you have used, i went around the car two times, using the automatic suction bleeder that uses venturi from the air compressor, i got a lot of fluid running, and yes i threw away lots of new fluid until stop

i used the blue ate fluid to make sure of getting rid of all the yellow one, yes i know, you can't cos it is not allowed in the us
 
i completely agree with deQ. often, you can get air into the lines from the fluid level dropping below the clutch line. I would go around and bleed everything - starting with the clutch, then to the front and rear wheel cylinders.
 
you can get air into the lines from the fluid level dropping below the clutch line.
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Brake fluid falling below the clutch port should not affect the braking system (unless you empty the reservoir completely).

If you manually bled then it is possible that you let air back into the system if your timing between pedal-pusher and bleeder-closer was off.
 
Arde said:
With the engine off the brake pedal is consistently firm.
Click to expand...

That symptom points to the booster, rather than to the MC. After all, the MC doesn't know whether the engine is on or off.

Should I just replace the master cylinder or spend more time diagnosing?
Click to expand...

I would diagnose more.

How long ago did you perform the bleeding? It is possible to get fine bubbles in the system - sort of like carbonation in champagne - which produces a spongy pedal. Those bubbles will dissipate after a couple of days as they rise to the top of the reservoir. So you might just leave the car alone for a few days and then see if the symptom is still there next weekend.
 
Yes Jay I noticed the symptom on the maiden voyage so it may be that I need to use the brakes more before taking action.

DQ and Stan point to the fluid itself and tangentially imply I used bile fluid instead of brake fluid. Maybe.

Inconclusive advise on master cylinder and clutch bleed. How can I exonerate the MC so I can move on?
 
come by the shop and you can try out my new pressure bleeder!
 
Just wondering if you know how old the MC is? Seems to me at this point there shouldn't be much original about our brakes.
 
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