bad brake fluid leak

356 Porsche-related discussions and questions.
Message
Author
User avatar
Mitch Covington
356 Fan
Posts: 200
Joined: Mon May 05, 2008 4:02 am
Location: Asheville, NC
Contact:

Re: bad brake fluid leak

#46 Post by Mitch Covington »

Thanks Doug, will check. I'm curious why, immediately after bleeding the system with the new master cylinder, the brake pedal was mushy, then a couple weeks later it was perfect, then a few weeks after that it was too much. The rod wouldn't be changing or getting tighter would it? Could the synthetic brake fluid be causing the rubber break hoses to swell and gradually close up?
Mitch Covington
Asheville, NC
'59 356 Cabriolet
'79 911 SC Targa

User avatar
Martin Benade
356 Fan
Posts: 12177
Joined: Wed Nov 23, 2011 10:52 am
Location: Cleveland, Ohio

Re: bad brake fluid leak

#47 Post by Martin Benade »

That’s a very good first step.
Cleveland Ohio
62 Cabriolet
56 VW
02 IS 300
04 Sienna

User avatar
Tom Wavrin
356 Fan
Posts: 470
Joined: Fri Mar 28, 2008 1:39 am
Location: Oregon

Re: bad brake fluid leak

#48 Post by Tom Wavrin »

This is an interesting read on comparison of DOT 3 versus 4 brake fluids published by Bendix and also about SBR versus EPDM rubber being used in brake hoses:

https://www.bendix.com.au/news-events/b ... d-issue-12
Tom, Reg # 10576, Oregon
'62 1600S Karmann HT

Dave Erickson
356 Fan
Posts: 2189
Joined: Sat Oct 26, 2013 9:34 pm
Location: Monterey, CA

Re: bad brake fluid leak

#49 Post by Dave Erickson »

The rubber hoses swell with age, shutting off the flow of fluid. I could imagine that leading to a harder feeling pedal. Just a guess.

User avatar
Martin Benade
356 Fan
Posts: 12177
Joined: Wed Nov 23, 2011 10:52 am
Location: Cleveland, Ohio

Re: bad brake fluid leak

#50 Post by Martin Benade »

I think the pedal pressure can generally force fluid in but the brake return springs are too weak to push it back out.
Cleveland Ohio
62 Cabriolet
56 VW
02 IS 300
04 Sienna

User avatar
Mitch Covington
356 Fan
Posts: 200
Joined: Mon May 05, 2008 4:02 am
Location: Asheville, NC
Contact:

Re: bad brake fluid leak

#51 Post by Mitch Covington »

Tom Wavrin wrote: Sat Mar 25, 2023 3:23 pm This is an interesting read on comparison of DOT 3 versus 4 brake fluids published by Bendix and also about SBR versus EPDM rubber being used in brake hoses:

https://www.bendix.com.au/news-events/b ... d-issue-12
Good article, thanks! They say: "DOT 4 brake fluid permeates the inner lining and then reacts with the outer PVA reinforcement braiding producing a viscous liquid which could build up between layers of rubber and makes the hose considerably weakened."

This would make sense that the Dot 4 caused my rubber brake hoses to swell and decrease the size of the hole, making it harder to pump fluid to the wheel cylinders. That would also explain why it was a gradual thing over a couple of months as opposed to a rod adjustment issue?

I have new hoses ordered, I think I will switch back to DOT 3 when I change them.
Mitch Covington
Asheville, NC
'59 356 Cabriolet
'79 911 SC Targa

User avatar
Mitch Covington
356 Fan
Posts: 200
Joined: Mon May 05, 2008 4:02 am
Location: Asheville, NC
Contact:

Re: bad brake fluid leak

#52 Post by Mitch Covington »

Dave Erickson wrote: Sat Mar 25, 2023 9:15 pm The rubber hoses swell with age, shutting off the flow of fluid. I could imagine that leading to a harder feeling pedal. Just a guess.
This would make sense. My issue happened pretty quickly but gradually in just weeks after replacing my master cylinder and putting DOT 4 fluid. It sure seems that is was the fluid that accelerated the swelling.
Mitch Covington
Asheville, NC
'59 356 Cabriolet
'79 911 SC Targa

User avatar
Tom Wavrin
356 Fan
Posts: 470
Joined: Fri Mar 28, 2008 1:39 am
Location: Oregon

bad brake fluid leak

#53 Post by Tom Wavrin »

Ate brake hoses are now made with EPDM rubber so DOT 4 would be the better choice of fluid according to that article. Bleed and replace it every couple of years. It might have been possible your old brake hoses might be composed of SBR rubber which, according to Bendix, could increase the risk of brake line failure in the presence of DOT 4 brake fluid. You might also consider putting new rubber kits in your brake cylinders while the hoses are off and fluid is out of the system.
Tom, Reg # 10576, Oregon
'62 1600S Karmann HT

User avatar
Harlan Halsey
356 Fan
Posts: 2338
Joined: Fri Mar 02, 2012 2:05 pm
Location: No Cal SF Peninsula

Re: bad brake fluid leak

#54 Post by Harlan Halsey »

Martin Benade wrote: Sat Mar 25, 2023 9:53 pm I think the pedal pressure can generally force fluid in but the brake return springs are too weak to push it back out.
Martin has it right: The braking pressure is far too high for any restriction to affect the braking. However the hose restriction acts like a check valve holding the brakes dragging which causes massive overheating. If your problem was hose restriction failure, it wouldn't affect more than one brake, not all of them. Second, brake hose restriction failure takes years, decades. Look elsewhere for the solution to your problem.

I have been using NAPA or Redline silicone brake fluid in all my drum brake cars for more than 20 years. I never have to change it. However, one of the hoses in my DOT 3 356SC did shrink closed and caused a fire and some excitement.

User avatar
Martin Benade
356 Fan
Posts: 12177
Joined: Wed Nov 23, 2011 10:52 am
Location: Cleveland, Ohio

Re: bad brake fluid leak

#55 Post by Martin Benade »

I suppose a pair of old hoses might fail in the same way at about the same time, especially on a car that sat for years. Way back when I knew very little, I had quite a time figuring out what was wrong with my BMW 2500. It turned out that three of my tires had shifted belts and a wobble. They still looked just fine.
Cleveland Ohio
62 Cabriolet
56 VW
02 IS 300
04 Sienna

User avatar
Mitch Covington
356 Fan
Posts: 200
Joined: Mon May 05, 2008 4:02 am
Location: Asheville, NC
Contact:

Re: bad brake fluid leak

#56 Post by Mitch Covington »

I went ahead and replaced the rear rubber brake lines. I've never seen the cross section of a new one... is this one (the old one) normal or constricted?
x-sect brake line Apr23.jpg
Mitch Covington
Asheville, NC
'59 356 Cabriolet
'79 911 SC Targa

User avatar
Martin Benade
356 Fan
Posts: 12177
Joined: Wed Nov 23, 2011 10:52 am
Location: Cleveland, Ohio

Re: bad brake fluid leak

#57 Post by Martin Benade »

That looks normal
Cleveland Ohio
62 Cabriolet
56 VW
02 IS 300
04 Sienna

User avatar
Mitch Covington
356 Fan
Posts: 200
Joined: Mon May 05, 2008 4:02 am
Location: Asheville, NC
Contact:

Re: bad brake fluid leak

#58 Post by Mitch Covington »

Thanks Martin. So unless the front ones are bad, then the hose swelling was not the issue. FWIW, I have disc on the front, drum in the rear. Would the next possibility be the wheel cylinder springs not being strong enough to push the fluid back out, as you mentioned above? Any other possibilities?
Mitch Covington
Asheville, NC
'59 356 Cabriolet
'79 911 SC Targa

Post Reply