Bad master cylinder?

[SteelGreyM]

Hey everyone. Car has been sitting for about a year as engine was being rebuilt.

Finally got the engine in. Runs great. But now I’m noticing the brake pedal doesn’t feel right. When the car is off the brake pedal is light at the top, has a brief moment of firmness in the middle, then light at the bottom.

When the engine is on the brake pedal is straight butter. However always rebounds.

I have a code for brake pressure sensor as well as yellow abs, TC, brake lights. However I think it may be the sensor acting up and reading the low pressure. I don’t think this will mess with how the pedal feels...

I’m not super familiar with braking systems. Maybe someone here knows what’s going on.

[heinzboehmer]

Did you open up the brake system at all when doing the engine stuff? I'd check to make sure things are properly bled, then go from there.

[eacmen]

1. Check for fluid on the ground and around the seals between the brake fluid reservoir and the MC.
2. To see if the MC is bad, with the engine off press the brake pedal a few times to remove the vacuum from the booster. Then apply constant pressure to the pedal. If the pedal continues to the floor with constant pressure applied then the MC is bad and needs to be replaced.

My gut is telling me that you just need to bleed the system or there is some intermittent vacuum leak that is affecting the brake booster performance. But I would think you'd be getting a lean condition if you had a vac leak.

[SteelGreyM]

1. Check for fluid on the ground and around the seals between the brake fluid reservoir and the MC.
2. To see if the MC is bad, with the engine off press the brake pedal a few times to remove the vacuum from the booster. Then apply constant pressure to the pedal. If the pedal continues to the floor with constant pressure applied then the MC is bad and needs to be replaced.

My gut is telling me that you just need to bleed the system or there is some intermittent vacuum leak that is affecting the brake booster performance. But I would think you'd be getting a lean condition if you had a vac leak.

It really is odd..

no fluid leaks anywhere. Clutch works great but brakes are spongy. When the car is off the brake pedal feels inconsistent. Light at the top and bottom with a brief moment of firmness in the middle...

If it is in fact the MC what would I need to replace exactly? Please dont tell me its this PN 34317838638

Did you open up the brake system at all when doing the engine stuff? I'd check to make sure things are properly bled, then go from there.

No not at all. Slave was unbolted but not undone. The clutch pedal works and feels great...

Didn't touch the calipers

[SteelGreyM]

Ok update.

When I push the brake pedal with the engine off I can hear noise coming from the area where the ABS and brake stuff sit in the engine bay. Noise is like a bad seal and air escaping. Kind of like a squeak.

Maybe this helps?

And yeah pedal is still spongy with car off no matter how many time I push it.

But the fluid level isn’t dropping. Stays full...

Read online maybe a sticking caliper pin? Is this possible?

[eacmen]

Ok update.

When I push the brake pedal with the engine off I can hear noise coming from the area where the ABS and brake stuff sit in the engine bay. Noise is like a bad seal and air escaping. Kind of like a squeak.

Maybe this helps?

And yeah pedal is still spongy with car off no matter how many time I push it.

But the fluid level isn’t dropping. Stays full...

Read online maybe a sticking caliper pin? Is this possible?

Pedal spongy with engine off is either air or MC.

The brake booster will sound like air escaping until its empty of vacuum.

Bleed the system then test again.

[liam821]

Yeah, I would bleed the system again. It took me a few times to get all the air out. Use a power bleeder too, it's a big help.

[SteelGreyM]

Will bleed the system on Sunday. Thanks for the tips guys. Hoping it’s just some stray air.

[SteelGreyM]

1. Check for fluid on the ground and around the seals between the brake fluid reservoir and the MC.
2. To see if the MC is bad, with the engine off press the brake pedal a few times to remove the vacuum from the booster. Then apply constant pressure to the pedal. If the pedal continues to the floor with constant pressure applied then the MC is bad and needs to be replaced.

My gut is telling me that you just need to bleed the system or there is some intermittent vacuum leak that is affecting the brake booster performance. But I would think you'd be getting a lean condition if you had a vac leak.

I re-read this and the idea of a potential vacuum leak has me thinking...

Where could this leak that effects the braking system be coming from?


The only thing I could think of is maybe this little hose that attaches to the air rail could have unattached without me noticing. I’ll have to check when I get home.

I am just in disbelief that the MC failed this badly this quickly. The more I think about it seems like a vacuum leak.

Then again, with the engine off the brake pedal goes to the floor easily... so probably not vacuum leak right?

[eacmen]

I re-read this and the idea of a potential vacuum leak has me thinking...

Where could this leak that effects the braking system be coming from?


The only thing I could think of is maybe this little hose that attaches to the air rail could have unattached without me noticing. I’ll have to check when I get home.

I am just in disbelief that the MC failed this badly this quickly. The more I think about it seems like a vacuum leak.

Then again, with the engine off the brake pedal goes to the floor easily... so probably not vacuum leak right?

Based on that statement its not a vacuum leak. But if there was one it really could be anywhere.

Only thing i can think of is that the brake fluid that was in there was old and caused the seals to degrade while the car was sitting.

[eacmen]

Just “for fun” (I totally didn’t forget to reconnect it) today i disconnected the vac hose going to the brake booster.

The pedal got super hard and the engine had erratic and lagging throttle response.

tl:dr a vac leak that affects the brake booster should be very noticeable!

[bigjae46]

Its either air in the brake lines or a bad MC.

[SteelGreyM]

Ordered the MC. Still in disbelief. Will update in a week

[eacmen]

Ordered the MC. Still in disbelief. Will update in a week

Yeah its really annoying ATE doesnt sell this in the aftermarket.

Let me know if it works for you op.

[SteelGreyM]

Yeah its really annoying ATE doesnt sell this in the aftermarket.

Let me know if it works for you op.

Will do. Starting to understand where “break my wallet” comes from...