Brake pressure problem

[adrian d]

Hello friends,
I recently ran into a very scary problem on my D. I was driving around as usual and the brakes felt normal, didn't catch my attention one way or another. Then after about an hour of driving (mostly on the freeway) I went to hit the brakes and the pedal went straight to the floor with almost no braking action. I tried pumping the pedal to see if it would "catch" or build up the hydraulic pressure, but it didn't work. For the rest of the drive home, the brakes were almost non-existent. I didn't get into an accident, but it was scary and closer than I would ever like to experience again in my DeLorean. I started it up again and drove around the block the next day and the brakes felt fine as if nothing had happened the day before.

There is no leaking from the master cylinder or any of the hoses or calipers. The pads are about halfway down, and the discs are fine.

Please help, I need to fix this before I can safely drive my D again.

Regards,
Adrian Dunker
10674

[Jonathan]

That doesn't sound like a fun drive home. Scary is right. I'd want to get that fixed ASAP too.

A couple ideas or things to check:

- take the cap off of the top of the brake fluid reservoir and tell us what you see. Level, colour of the fluid, consistency, any debris in it, etc.?

- if the level looks fine and they went from being non-existent to being ok again, I would suspect an air pocket somewhere. It won't need to be an air pocket in all four brake lines, just one line with a big enough air bubble that all the force of the pedal pushes on the compressible air that got in there. So, to check or fix that, you should bleed each brake starting with the ones furthest away from the master cylinder, i.e. rear right, rear left, front right and finish front left. Then you ensure again the fluid is topped up or otherwise maintains a good level.

- another possibility might be one or more of your flexible lines at each caliper somehow deteriorated enough to swell and then the pedal force is pushing the fluid out and making the hose wider/swollen but not actually pushing down the length of the line to the caliper. I've heard manual trans drivers talk of this happening with clutch lines, so maybe something similar has happened to your soft lines in the brake system?

Check fluid situation first at reservoir, then bleed each brake line at caliper, then re-evaluate.

[David T]

That doesn't sound like a fun drive home. Scary is right. I'd want to get that fixed ASAP too.

A couple ideas or things to check:

- take the cap off of the top of the brake fluid reservoir and tell us what you see. Level, colour of the fluid, consistency, any debris in it, etc.?

- if the level looks fine and they went from being non-existent to being ok again, I would suspect an air pocket somewhere. It won't need to be an air pocket in all four brake lines, just one line with a big enough air bubble that all the force of the pedal pushes on the compressible air that got in there. So, to check or fix that, you should bleed each brake starting with the ones furthest away from the master cylinder, i.e. rear right, rear left, front right and finish front left. Then you ensure again the fluid is topped up or otherwise maintains a good level.

- another possibility might be one or more of your flexible lines at each caliper somehow deteriorated enough to swell and then the pedal force is pushing the fluid out and making the hose wider/swollen but not actually pushing down the length of the line to the caliper. I've heard manual trans drivers talk of this happening with clutch lines, so maybe something similar has happened to your soft lines in the brake system?

Check fluid situation first at reservoir, then bleed each brake line at caliper, then re-evaluate.

All of that sounds good but the master cylinder is bad and you can't trust it anymore. It must be rebuilt or replaced before you drive the car again. Air pockets do not come and go.

[Ron]

If the fluid level is up and you don't get any air when you bleed them, odds are it was the master cylinder bypassing internally.
You might reproduce it by pressing the peddle down very slowly with short soft pumps...

[DMCVegas]

All of that sounds good but the master cylinder is bad and you can't trust it anymore. It must be rebuilt or replaced before you drive the car again. Air pockets do not come and go.

Agreed. A new Brake Master Cylinder is $176 from DMCH. If you wrecked the car tomorrow because of this and someone offered you the once-in-a-lifetime chance to repair all of the damage and injuries resulting from it for a measly $176, you'd jump at that opportunity.

So why not just pay that now, and avoid the danger all together?

[Ron]

FWIW
Rock Auto sells a MC for ~1/2 that price - Saab 90 (NOT 900) with Lucas-Girling brake system. 63/37.
I have NOT tried one myself, but a Dorman CMK351685 rebuild kit is reported to fit. <$20.00
I'd use the original springs to ensure 63/37.

[Andrew]

Sounds like the internal seals of the brake master cylinder. I've lost the master cylinders on a number of my cars over the years, including both of my DeLoreans, my former Bricklin and 2 other cars. The symptom was always the same. Sudden brake failure with no visible fluid loss. If it's not leaking fluid anywhere, and the car was braking evenly when you push the pedal otherwise, I would DX it as a master cylinder failure.

BTW I'm running a Saab 900 unit on 5052 and it works great! I can't tell any difference between the braking of 4194's stock replacement and 5052's Saab unit.