Braking issue - is it my master cylinder?

[Lorin]

When I first touch the brakes whilst driving, the pedal is soft with a LOT of travel. Not quite to the floor but close. The brakes still work but the pedal has to be pushed a long way. It is quite disconcerting. If I just let off the brake pedal and reapply, they are perfect again with minimal travel and braking almost immediately. If I hold the brakes on there is no pedal creep. With the engine off the pedal remains firm and there's no creep.

There's no fluid leaks that I can find and I haven't fiddled with anything that could have affected them. The reservoir is at the full level.

Is this likely to be the master cyclinder or something else? Just off on hols for a week driving the in the truck so would like to get it sorted asap.

 

[Chris]

Typically I would say it needed bleeding, but as you say you haven't been twiddling. There isn't a great deal it could be. Master, caliper, hose, vacuum booster. Have you tried a quick bleed all round Lorin?

 

[Lorin]

Cheers Chris. Nope I've done nothing. I'll pop it into a garage I know and get a full bleed done.

 

[Dave2000]

As per Chris's post but would add, in the absence of 'twiddling' have you checked for play in the wheel bearings? When driving if the bearings are loose enough you get a phenomenon known as 'pad knock off'. The discs rocks in the calliper and 'knocks' the pads and pistons back into the calliper, the first press of the pedal brings the pads out to the correct position and the second press you have brakes.

regards

Dave

 

[Lorin]

Dave, I haven't done anything yet. That is something I can easily check though and will do

 

[Chris]

Nice learn there Dave. Have to say I haven't come across that one before. But filed away for later.

I don't see HOW it could be air really, but bleeding is so easy that you really have to do it. Just to eliminate the most basic. Lorin, you may have read posts in the past about bleeding using the pedal on the old trucks. If you push the pedal too far you can mangle the seals in the MC. Much better to either pressure or vacuum bleed the brakes - just before your chap goes pumping the pedal.

 

[joinerman]

Think Dave's spot on, i had exactly the same symptoms, nipped the bearings up & perfect again.

 

[Chris]

Have to say it makes perfect sense

 

[frank rabbets]

A common symptom of play in the wheel bearings. With the fronts especially noticeable after maneuvering round a car park.

 

[Lorin]

Dave's suggestion fits my symptoms well, as the pedal only goes soft if the wheels have moved. If I stay static the pedal remains firm. We've reached our destination tonight, so Monday I will nip it to garage and check the bearings and do a bleed. I've always shied away from bleeding with the pedal for that very reason Chris, pressure or vacuum bleed only

 

[Ken Brenton]

Same symptons some years ago on my 80 GXL whilst on 2 day trail in RSA. The Cruiser whisperer on LCCSA recommended checking wheel bearings and sure enough play on drivers side, not extensive but after adjusting braking problem resolved.

 

[frank rabbets]

The wider the tyre the easier the pads are pushed back. When stationary and you turn the steering king pin and calliper turn but the wheel and disc lag behind until the play in the gearing is taken up. By this time the pads are pushed into the calliper. Mine did it from day 1 but it was 3 years or so until I bothered sorting it.

 

[Lorin]

Excellent bit of knowledge that Dave When I checked the wheel bearings, DS was fine but PS was so loose I could turn it by hand. Nipped it up and the brakes are back to how they should be. No bleeding necessary which is always a bonus too.

 

[Dave2000]

Your welcome mate.

regards

Dave

 

[Chris]

Great result. Well done. Never come across that as I said, but maybe because I have never had a wheel bearing loose like that.

Periodic bleeding is still good though. You get all sorts of filth work its way into the caliper.

 

[Dave2000]

Agree Chris, if anyone is unsure why then here is the reason,

Brake fluid is hygroscopic, in other words it absorbs moisture/water. Water works it's way through the system to the lowest point (heavier than oil) and this is the wheel cylinders/callipers, they then rust and can fail.

If the fluid is NOT changed but have not yet failed, and you have great brakes (water is not compressible) then you may find the first time you REALLY need them (long steep hill/caravan/family etc) then the brakes get very hot, the water in the callipers turns to steam (compressible) and the pressure at the disc/pad face falls, car gains speed and you press harder, ad infinitum.

If the brakes fail and if you survive and explain the brakes failed, the police officer checking the pedal will say nothing wrong with them sir/madam, pedal is good and solid!

The steam has returned to water and restored the aforementioned 'great' brakes, so worth a bleed anyway if history is unknown, block of wood under the pedal stops it travelling too far, thus avoiding possible damage to the master cylinder during bleeding, or pressure/vac bleed.

regards

Dave

 

[frank rabbets]

I have used silicone fluid in my braking systems some years ago. This does not absorb water and does not strip paint off the callipers. Unbeleivably though it finds it's way through joints which ordinary fluid cannot leak through. Not on brake pressure per se but just standing overnight. I resumed work on restorations in the morning to find moist joints all over the place never having had any such problem before.