Yet more 404 brake weirdness.

Gilow

Member
Fellow Frogger
Joined
May 21, 2015
Messages
130
Location
Adelaide
Hi all,

The 404 brake saga continues. To recap, 1968 or 69 disk brake model.

I’ve had the master cylinder done, I’ve fitted a new booster, I’ve replaced the limiting valve on the rear axel with one of those adjustable limiters used in racing, a good one purchased from PBR in Adelaide, and it sits between the solid pipe on the chassis and the flexible hose to the tail shaft. The rear limiter has been replaced with a three way union, no moving parts.

Rear slaves were replaced about three years ago, but were not used for a year or more while we waited for my son to be old enough to drive the car.

Now the car keeps getting air in the lines, in the back half of the circuit. Clamping the hose between the limiter and the drive shaft immediately fixes the brakes to rock solid.

There is absolutely no evidence of fluid loss in the system. But within a week or two of driving the brakes get softer and softer and softer.

The only idea I can come up with is that it is somehow drawing air back in through the seals on the rear slaves. Is this possible?

Matt
 
Matt,

Maybe the flexible hose has a small fracture in it. If it is the original rubber line it is highly likely, thus allowing air into the circuit. Albeit a small amount. In the past, I over clamped the line and it failed internally.

Not questioning your knowledge, but have you also removed the drums and peeled back the cylinder dust caps. It could be an internal rubbers are weeping and any lost fluid could be in the dust cup.

Cheers,

Dano
 
The brakes on my (now long departed) 404 never gave any trouble, but I'll offer a generic suggestion.

If you think about the way around the cups are in the wheel cylinders, they're ideal for letting air in. What keeps it out is hydraulic pressure which should never get down near atmospheric pressure. That's the reason drum brake systems have a "residual line pressure valve" (typically to keep something over 5psi in the brake line). Worth checking if you can't find any leaks in hoses or connections.

Have fun,

Rob.
 
The brakes on my (now long departed) 404 never gave any trouble, but I'll offer a generic suggestion.

If you think about the way around the cups are in the wheel cylinders, they're ideal for letting air in. What keeps it out is hydraulic pressure which should never get down near atmospheric pressure. That's the reason drum brake systems have a "residual line pressure valve" (typically to keep something over 5psi in the brake line). Worth checking if you can't find any leaks in hoses or connections.

Have fun,

Rob.

Rob, do you remember where the residual pressure valve is located in the 404?
 
Matt,

Maybe the flexible hose has a small fracture in it. If it is the original rubber line it is highly likely, thus allowing air into the circuit. Albeit a small amount. In the past, I over clamped the line and it failed internally.

Not questioning your knowledge, but have you also removed the drums and peeled back the cylinder dust caps. It could be an internal rubbers are weeping and any lost fluid could be in the dust cup.

Cheers,

Dano

Thanks Dano, I should have mentioned that I have replaced all of the flexible hoses. The dust cup idea is interesting, I’ll check, but the fluid level is very constant.
 
I experienced this same problem with a 504 wagon i had ,had the rear wheel cylinders re sleeved locally ,i ended up getting another re sleeved set from Lewin Partridge in bris .my conclusion was that the local guys had fitted imperial size rubbers ,which were smaller and flexing during operation allowing air in ,they could have sleeved it an imperial size and fitted metric seals ,wayne at better brakes southport who you have dealt with ,found me a set of new cylinders complete a while back ,they weren't all that expensive ,I THINK THE VALVE IS IN THE 3 WAY SLITTER ON THE FRONT CROSS MEMBER ,LEFT HAND SIDE WERE THE LINE CONNECTS TO GO TO THE REARS ,i feel for your son ,it is relay disconcerting when the pedal disappears ,ps my conclusion was that the seal expands when hydrolic pressure was put on it ,but flexed enough to let air in as it returned ,same as you ,it took a while but the pedal slowly got worse ,pugs
 
Rob, do you remember where the residual pressure valve is located in the 404?

Afraid not -- that's why I called it generic advice. Here comes some more.

It's single circuit brakes, so the valve won't be in the master cylinder like most modern cars. It's probably going to look like an extra "unnecessary" union (e.g. here is a modern in-line valve -- don't expect yours to be racing red). I'd look for it somewhere in the line after the proportioning valve. It *might* be before, but after makes more sense to me.

Mind you, I'm not sure how to test the valve itself. Maybe trying reverse pressure on a regulated compressed air line and dial up the regulator until the valve opens. Pretty fiddly.

The other check (perhaps when you're checking for fluid at the dust seals) is to be sure that the push rods at the cylinders are firm and haven't pulled back from the shoes at all. Probably best to check this after coasting to a stop (handbrake would be ok) after a suitably vibratey drive.

Have fun,

Rob.
 
In the 1 in 3 out union on front cross member. The one to the rear will look slightly different. If it is a 1968 model it won't have this valve. Has the car got door opening levers down near the arm rests?
 
Hi Matt,

Another place you may want to check is the master cylinder. A 404 master cylinder leaks back into the car via the pedal assembly mounting plate. If it is a slow weep, there will be a mist of fluid on and around the firewall where the brake pedal shaft enters the cabin.

As a side note, this one of the reasons 404 driver side floors rust. Over prolonged periods of time, the leaking brake fluid eats away the paint and then any water ingress starts the rusting process. The fluid also absorbs into the fibrous sound proofing mat, which compound the issue. Until it becomes a full on flow (?) one may not smell the fluid.

It is just a thought, and on rare occasions new parts do fail. We never now how long our new parts have been sitting on a shelf in some warehouse.

Cheers,

Dan
 
In the 1 in 3 out union on front cross member. The one to the rear will look slightly different. If it is a 1968 model it won't have this valve. Has the car got door opening levers down near the arm rests?


does this photo help?
9B239EE8-8427-468A-B175-41C5477CDED9.jpeg
 
Hi Matt,

Another place you may want to check is the master cylinder. A 404 master cylinder leaks back into the car via the pedal assembly mounting plate. If it is a slow weep, there will be a mist of fluid on and around the firewall where the brake pedal shaft enters the cabin.

As a side note, this one of the reasons 404 driver side floors rust. Over prolonged periods of time, the leaking brake fluid eats away the paint and then any water ingress starts the rusting process. The fluid also absorbs into the fibrous sound proofing mat, which compound the issue. Until it becomes a full on flow (?) one may not smell the fluid.

It is just a thought, and on rare occasions new parts do fail. We never now how long our new parts have been sitting on a shelf in some warehouse.

Cheers,

Dan

thanks Dan, but the master cylinder has been done. And I’ve checked everywhere, there is no fluid loss, and this is born out by the reservoir remaining constant.
 
In the 1 in 3 out union on front cross member. The one to the rear will look slightly different. If it is a 1968 model it won't have this valve. Has the car got door opening levers down near the arm rests?

here is the union...
09A6779B-B56E-4BC2-8F10-E0D10321FFFB.jpeg
 
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