No brakes...help

[67 Coronet Conv]

interesting situation...

1967 Coronet with power disc brakes up front and drums in the rear. 440 with a cam, previous owner added external vacuum pump for the power brakes.

Everything was working fine on a drive this past weekend and pulled into a parking lot and the car useless for about 5 min (talking to some guy about the car). When I left I had no brakes, like I was pushing on it really hard and it barley slowed down....didn’t even feel like manual brakes. I pull over, pump the brakes and still nothing. Turn the car off then on to see if I can here the vac assist pump up and can’t hear it. I wait a little bit and brakes come back and I drive real slow home. Pull into the garage and look under the hood and the master cylinder is leaking from the lid like a drop every 5 seconds and it made quite a mess.

Any ideas on why I intermittently lost my brakes?

Thanks InAdvanced
J

 

[Jerry Hall]

interesting situation...

1967 Coronet with power disc brakes up front and drums in the rear. 440 with a cam, previous owner added external vacuum pump for the power brakes.

Everything was working fine on a drive this past weekend and pulled into a parking lot and the car useless for about 5 min (talking to some guy about the car). When I left I had no brakes, like I was pushing on it really hard and it barley slowed down....didn’t even feel like manual brakes. I pull over, pump the brakes and still nothing. Turn the car off then on to see if I can here the vac assist pump up and can’t hear it. I wait a little bit and brakes come back and I drive real slow home. Pull into the garage and look under the hood and the master cylinder is leaking from the lid like a drop every 5 seconds and it made quite a mess.

Any ideas on why I intermittently lost my brakes?

Thanks InAdvanced
J

What is the fluid level in the MC? Extremely hard petal could be vacuum pump, brake booster or probably not a bad MC. If you were able to hear the vacuum pump run before the problem, it's probably there. If you can pull the vacuum line off at any place and hear a vacuum rush then your vacuum is working. With the vacuum line disconnected at the booster, if the vacuum pump runs your pump is good. A problem I had recently was very similar to yours. I have no vacuum pump, but the petal became very hard and I almost had to put both feet on the brakes and all my weight to get it to stop. Mine was easy. With the engine running I pulled the vacuum line off the booster and no sound. Worn out brake booster. Also make sure your the rubber seal on the MC is good. The extreme pressure you had to apply to the brake pedal could have made the MC leak a little.

 

[67 Coronet Conv]

MC had fluid in it. I’ll check the line to see if it has vacuum.

This still wouldn’t explain the MC getting pressurized and start to leak out of the top.

 

[moparedtn]

MC had fluid in it. I’ll check the line to see if it has vacuum.

This still wouldn’t explain the MC getting pressurized and start to leak out of the top.

Jerry offered a possible explanation in the very last line of his response....

 

[dadsbee]

Happens again go feel all your wheels. Possible you have a brake that's badly overheating and boiling a wheel cylinder / pistons fluid = no brake pressure like my '86 Southwind coming down the East grade out of Yellowstone in 2004!

 

[Frank Mopar]

You should not get any pressure in the mc. When you step on the brake the plunger in the mc closes off the fluid port so if you are the mc is probably bad.

 

[Ron H]

When you mention pushing hard on pedal was the pedal sinking down to or near the floor before you had any glimpse of braking? There are a couple of simple checks you can make on the booster with a vacuum pump/gauge with engine off and running to determine its health. You say you didn't hear the aux vac pump at all? Are you hearing it now? The MC leak is odd here and of course check to ensure the gasket is undamaged and set right also the lid sits solidly tight after latching it down. A hard pedal can be a sign of air in the system and also could include the MC. There are on and off the car MC bleeding but highly suggest a bench bled if the MC looks to be functioning properly. Aftermarket disk brake conversion? If so, there can be some gremlins if not done right...among them is a lousy MC bleed before installing.

 

[Don Frelier]

A hard pedal can be a sign of air in the system and also could include the MC

I have to disagree with this one, I've always found air to give a soft pedal.
I've never seen a working master cylinder and brake system suddenly need to bleed the master?
He lost his assist for some reason.

After this episode, I would be seriously considering a manual master cylinder and get rid of the other stuff.
I have manual discs in the front with drums in the rear on my Charger and it stops pretty well.
But it does require some pedal pressure.

 

[67 Coronet Conv]

Update-Put a gauge on the vacuum line and am getting 21in vacuum. So i think its time to replace the MC..Any recommendations on a MC?

 

[67 Coronet Conv]

So i replaced my MC and that stopped the leak, but still was losing vacuum. So I drove the car around and when it lost its power breaking ability (aka-Vacuum) i pulled over and stared testing the wiring with a volt meter and found that the power wire wasn't spliced correctly and fixed it. Headache solved.

 

[Ron H]

So i replaced my MC and that stopped the leak, but still was losing vacuum. So I drove the car around and when it lost its power breaking ability (aka-Vacuum) i pulled over and stared testing the wiring with a volt meter and found that the power wire wasn't spliced correctly and fixed it. Headache solved.

Could you describe the power wire you repaired? This is a new one on me with regard to vac loss.

 

[Jerry Hall]

Could you describe the power wire you repaired? This is a new one on me with regard to vac loss.

Me too.

 

[Ron H]

I see in his post there is an external vac pump on the car so must a been that wire..

 

[67 Coronet Conv]

I have an external vacuum pump for the brake power booster that uses 12v to power it. The location where it was pulling power from was not making contact.

 

[Jerry Hall]

I have an external vacuum pump for the brake power booster that uses 12v to power it. The location where it was pulling power from was not making contact.

Makes sense now, thanks for the info.

 

[Ron H]

I have an external vacuum pump for the brake power booster that uses 12v to power it. The location where it was pulling power from was not making contact.

Can I ask ya to let me know if your problems sounded similar to mine as I'm hunting down a brake problem that's driven me bonkers for some time: My brakes overall work fine except my pedal will seep down, but will still have brakes when this happens. Just that pedal is way down and brake lights will come on. While pedal is near to floor I can push it further and have brakes. I put my foot behind pedal and pull it up, pump a couple times all good...till the next time. My car has a power front disk brake conversion. I did several recommended vac checks on the booster with a vac pump and each one checked out exactly as supposed to be. I separated the boost from the MC and return action is fine snaps back to home. I got a decent deal on a reserve reservoir so put that in and no diff. My car has a slight cam and I have a vac gauge in the car and shows good vac cruising along. Wondering though if I could need an external pump. Thanks

 

[Jerry Hall]

Master cylinder maybe.

 

[Ron H]

Master cylinder maybe.

Ya know how you try to connect the dots on a problem having some understanding of how all is supposed to work and in what sequence? I was wondering about the MC maybe needing a bench bleed in all the gremlins I've tried to hunt down. What confuses me here though is why the pedal seeps down and it stays down since there's a return spring in the booster that should have the pedal come back to top say despite an MC issue? Even if the MC might be malfunctioning, da pedal should come back...so I'd think. The logic as I get it (qualify not a brake guru) is this should be a bad booster or inadequate vacuum right? I even put in a return spring as an idea and be damned if the pedal still stayed down. I'm sure I'm missing a few bolts in my head on this and have read dozens of articles on the web. Could be some incapability thing with the system...

 

[Ranger16]

If the pedal slowly sinks to the floor and there are no external leaks, it almost has to be a bad seal on the MC piston. The pedal SHOULD return to the up position, but I'd still bet on the MC.

 

[67 Coronet Conv]

Sounds like the MC, maybe bench bleed it or replace it.

how much vacuum are you getting at idle? My intake pulls about 14, where my external vacuum booster pulls 24