Intermittent brakes

[Mason]

Hi everyone,

I've been working on my 1964 Triumph Herald for a while now, and I'm very near the point where I can finally take it out for a spin, but I am having a problem with my brakes.

I have bled the system multiple times, and there is no air in the lines. When I stamp on the brake pedal, I have brake pressure, but when I slowly depress the pedal, it goes straight to the floor, no pressure whatsoever. 

I have fitted a set of refurbished spitfire disks and calipers, making sure to check that the bleed nipple on each caliper is at the top, refurbished the master cylinder and completely redone the rear brakes, as well as replaced the line running from the master cylinder to the junction splitting the line to the front and rear brakes. 

If anyone has any ideas as to what might be causing this, I would be hugely appreciative.

 

[Hogie]

Hi Mason,

air in the system would have the same effect whether pressed hard pr soft.

I would suggest that you have a dodgy master Cylinder/seals.

Is it a new item (if so refit the old one and see what happens)

If an old unit did you replace the seals (the right way round !!)?

 

Roger

[RobPearce]

As roger says, air in the system would not cause those symptoms - any difference would be the other way round. My bet's on the master cylinder seals.

If you stamp on the pedal, then hold your foot on it, does the pressure hold or does the pedal gradually sink? If the former, it's likely the fluid return seal (the little one) not seating right. If the latter, it's probably a split in the seal. It's not likely to be the main seal or corroded bore, because either of those would cause fluid loss.

[Mason]

I'll have a look when I get home and get back to you. I've never had any issues with the work from the company who refurbished the cylinders, but everyone makes mistakes.

Thank you so much for this insight. I'll keep you posted.

[Nick Jones]

Quoted from Mason-

When I stamp on the brake pedal, I have brake pressure, but when I slowly depress the pedal, it goes straight to the floor, no pressure whatsoever. 

If anyone has any ideas as to what might be causing this, I would be hugely appreciative.

 

Assuming you have no external leaks then the only possibility is that it is the small internal seal in the master cylinder that is supposed to seal off the fluid reservoir from the cylinder itself.  Either seal itself (it's tiny) has a defect or the surface it acts against.  Could even be a little bit of dirt.  It's definitely the issue though.

Nick

[Bitumen Boy]

I'll echo the thoughts of a dodgy valve seal in the mater cylinder. My experience of these is that while the seals may well be good and recently renewed, the various small parts that support the valve seal can wear imperceptibly to the point that they simply don't do their job reliably any more and the only lasting solution is a new master cylinder.

Although it seems to be a problem that more often affects the clutch m/c - presumably as they get a lot more use - there's no reason it can't affect the brake m/c in the same way. Indeed, I think I'm right in saying that the cylinders are identical on a Herald so your brake m/c may have once been a clutch cylinder... do yourself a favour and bin it.

[Mason]

Update time. I stripped the master cylinder and sent it back to the refurbisher. He got it back to me the same day, apologising profusely. Apparently he had a defective batch of grease which he used, causing the seal to swell. I put it back together last night, and it's working perfectly. 

 

Thank you all for your ideas and support. I'll be taking her out for the first time this weekend!