Hard braking goes weak

[John_Mc]

My brakes normally seem to function just fine. But if I hit them hard, there's a pop and they go weak. By quickly letting off and hitting them again (lighter) the pressure is back. But I'd really rather not have that happen at all, of course! My PDWA switch is broken. Is this a symptom of a pressure difference? How do I resolve this? Where do I begin? I'm guessing I probably just need a good bleed, but is there something more?

 

[AltaKnight]

Not sure what you mean by goes weak, does that mean pedal goes to the floor or do you mean lots of effort and not much action?
If you mean the latter I suggest it's likely the brake vacuum booster is failing.
Of course "having a good bleed" as you put it might also create this symptom but my 1st guess is the booster.

 

[John_Mc]

by going weak, I meant that after the pop, the pedal goes essentially to the floor and the brakes let up a bit. So I can't push any further, but I'm still not stopping. As soon as I let up and push again, pressure returns and I can stop okay. Not too safe, epecially in an emergency. Normal braking is fine. It's just when I put hard pressure on it abruptly.

 

[Alan_Myers]

Hi,

I am not all that familiar with the DPWA switch, but the principle just seems to be that it "senses" when the balance of front and rear brake circuits are imbalanced and sends a warning. It appears to need to be centralized after work is done on the brakes. It could be that one circuit is bleeding off into the other. However, I don't think that would necessarily cause the symptoms you describe. Still, you might want to fix the PDWA, just to be sure.

I think the most likely suspect is the master cylinder. If the rubber plunger seal in it is faulty, it might work up to a point, but give way under heavier pedal pressure. To some degree, that might happen also within the calipers or the rear slave cylinders, all of which have similar seals. I would suggest checking each closely for any signs of leaking fluid. Also check the fluid reservoir, if fluid is getting low, and especially if it's already gotten low enough to allow air into the system, that would indicate fluid is being pushed out somewhere. Even if fresh fluid isn't found at any of them, you might observe the master cylinder, each caliper and each rear slave cylinder while someone puts heavy pressure on the pedal, to see if you can see some fluid being forced out.

That's another possiblity, there could be air in the system causing the symptoms you describe. This happens when there is a leak somewhere in the system or the fluid in the reservoir at the master cylinder is allowed to get really low. Proper bleeding might solve the problem temporarily, but you still need to figure out and fix whatever is causing the fluid to drop in the reservoir, wherever it's escaping from the system.

Recent work on brakes might also leave air in the system, requiring additional bleeding.

Most brake fluid should be changed every 18 months, even if a car is used only occasionally. Brake fluid absorbs moisture from the atmosphere and that can in turn cause corrosion inside the system. That corrosion can lead to leaks and failed seals. If it's been a while since the fluid was changed, might be a good opportunity to do so.

Also, are you using DOT 5 (silicone) brake fluid? Some folks report problems using it. It's particularly hard to get into the system without inducing a lot of air that is can be difficult to bleed out later. Yes, it's better for the car's paint, but I know, for example, that MGB owners have traced some problems with their cars' vacuum assist/booster to a recent switch to DOT 5. And, it sometimes gives a softer pedal (although not a sudden failure like you are seeing) in various cars. Finally, I've read that it tends to globulize any moisture, which can turn to steam and cause brake failure when the brakes are used particularly hard (as in driving the car hard/racing, not just an occasional "panic" stop). As you can probably tell, I'm not a big fan of DOT 5. I've never used it in any of my cars. But, to be fair, others like it and use it, seemingly without problems.

As Graham noted, it doesn't sound like a problem with the vacuum assist/booster. Usually that just feels like the brakes take a lot more pedal pressure to produce the same results, but the car can still be stopped properly. The booster "multiplies" your foot pressure on the brake system by nearly double. That also means the system can see pretty high pressures at times.

If you can't identify the problem quickly, I strongly suggest taking the car (very carefully) to a pro right away. Brake problems aren't something to be taken lightly, for all the obvious reasons. If you do have a catastrophic hydraulic failure, keep in mind that the parking brake is activated by separate cables and will still work (although the parking brake on TR4A through TR6 isn't the greatest).

Take care!

Alan Myers
San Jose, Calif.
'62 TR4 CT17602L

 

[John_Mc]

Thanks for the suggestions Alan and Graham! I'll check those items. I did replace the seals on a rear cylinder a while back and may not have effectively gotten all the air out of the lines, especially since I did it myself with a hand actuated vacuum pump. I'm not convinced I was ever able to get a good enough seal to ensure I had bled it all the way. I 'll cover all the bases as you mentioned though.

 

[Guest]

John, to the best of my knowledge, a new PDWA switch for the TR6 is not available. They, of course, can be repaired and/or rebuilt satisfactorily. However, if this is not doable, a good alternative is to use the same switch from a Spit, which is available new. These, however, are somewhat larger in size the OE TR6 switches and may not link up well to the steel brake lines in your car. I changed all my braking lines to the nickle/copper aftermarket with good results. Going is good/stopping is better!

Bill