Brake pedal goes soft, can be pumped back up, diagnosis? (1 Viewer)

[IanB]

This started yesterday, go for the brakes and the pedal has resistance, then the bottom just drops out and the pedal needs pumped to get brake power back. Once pumped up the pedal is firm and holds pressure, but once in a while the pedal just goes soft and needs pumped. Is this a failing master cylinder? Brake booster? Check valve?

The brakes were flushed with fresh fluid and bled at the end of August, and had a very firm pedal up until yesterday when this started. Any help is appreciated!

 

[DARKNESS]

When it drops do you lose braking power?

 

[IanB]

When it drops do you lose braking power?

Yes, you lose brake power when the pedal goes soft and pushes towards the floor until you pump it back up.

 

[jkilcullen]

Likely its a master cyl, autozone sells a great aftermarket that includes a bench bleed kit for about half of dealer if I remember correctly.

 

[DARKNESS]

I would guess master cylinder. Bad booster would give you a hard pedal but still have braking power if you push really hard.

 

[IanB]

That was my suspicion as well, looks like it's a RHD specific part, might have some luck tracking down a used one fairly quickly, but a new one is going to be a few weeks or better, doh!

Is there a way to test it's a bad MC for sure before I start throwing parts at it?

 

[DARKNESS]

If you can get it to do it while parked with the hood open you might hear noise in the master cylinder or see turbulence in the fluid in the reservoir.

 

[Onefastrx7turbo]

If its too much of a pain in the ass to find another RHD one, you can rebuild the master cylinder pretty easily. I rebuilt the one on my RX7 with an overhaul kit from Mazda.

 

[Seth Derrick]

Check your pads first....seriously.

 

[Golgo13]

Pumping soft things is a job for @NLXTACY ...

Nobody else caught that in the title? Really?

What is the part number for the RHD master cylinder?
You might be able to just use a LHD and make your own lines from the master.
Rebuilding the seals yourself isn't a terrible job. that shouldn't matter RHD or LHD.

 

[NLXTACY]

Pumping soft things is a job for @NLXTACY

It was one time! All you had to do was say my name and look up at me with those approving eyes. But no, you insisted on wearing that hideous blue eyeshadow and it reminded me too much of Mimi from the Drew Carey show. Come on man, at least try.

 

[IanB]

Check your pads first....seriously.

Pads had plenty left in them when I checked them in September, I'll have another peek but I'm confident they aren't the issue. Looking into a used MC or a rebuild kit.

 

[SmokingRocks]

master cylinder all day. Easy diagnosis. Just rebuild or replace with aftermarket cylinder.

 

[IanB]

Ordered a new MC from Milner's Offroad in the UK, tried to go through Akella but haven't heard anything back from him, which is odd. Hoping it's here for next weekend...winter has struck and I'm really missing the truck.

 

[BCR4619]

Humm, he is around as he posted on Instagram this morning.

Hopefully that fixes things! I had a soft pedal at one point but it kinda fixed itself after I changed my seized rear callipers....

 

[IanB]

Well the MC I ordered from the UK was not the right one (my fault), so I got new OEM one last night and installed it, bled the brakes, and my issues are solved! Truck had been down for nearly a month, I really missed it.

 

[Pippylongsausage]

Mine did this and it turned out I had a hard brake line that had rusted through and was leaking very slowly under hard braking.