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Why brakes and hydraulic clutches lock up when they get hot

1828 Views 14 Replies 9 Participants Last post by Chuckracer, 2 mo ago

TUNA

Discussion Starter · 2 mo ago

Complaints about the rear or front brakes locking up has been discussed for years. Hydraulic clutches will also do the same when they get hot. Here’s what really happens. For whatever reason, the hydraulic fluid gets very hot and begins to expand. If you have a fancy smanchy fluid reservoir, it must be a vent hole to give the small amount of air under the reservoir cap somewhere to go. If the tiny amount of air is trapped between the fluid and the cap, the air has no place to go. Technically this is the rubber air bladder’s job. But if it gets hot enough it doesn’t work. As the fluid and air gets hot it expands and pushes against the hydraulic fluid. Thus applying the brakes or causing the clutch slave cylinder to slowly disengage the clutch, which results in the clutch slipping.
To confirm this on your bike, remove the reservoir in question and drain its fluid. With the cap and rubber bladder installed, blow into the hose fitting with your mouth. The air must escape. If your reservoir is air tight, there’s your problem.
You may have damaged your caliper, master cylinder, or clutch disc when they locked up, so you may have to replace them. But most of the time they’re still ok.
In the rare case a properly vented reservoir still doesn’t fix the problem, connect a power bleeder to the caliper or clutch slave cylinder. With all the fluid in the system, and everything else in the neutral position. With the reservoir cap on tight, open the bleed screw and back bleed the system. Brake fluid should come out the vent in the reservoir cap. If it doesn’t, you need to find the restriction. Maybe the master cylinder doesn’t have any free play. If the reservoir cap is vented and on tight, brake fluid must move into the reservoir as air is pushed out the vented cap.
I don’t have to tell you brakes locking up is very dangerous and a good way to get killed.
If you really cooked the system, you may have to replace the parts. Over heated parts can intermittently stick, but as long as the cap vents the system they shouldn’t lock up.
mad a footnote, Castrol React is excellent ultra high temperature brake fluid. Change the fluid in the Spring and Fall.

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TUNA

Discussion Starter · #2 · 2 mo ago

Of course I forgot to mention a couple of things. (Mr Magoo)
If you’ve gotten the calipers super hot, the pistons and the body of the caliper can warp to the point they won’t release until they cool down. Even though you fix the vent problem the pistons may intermittently stick with hard use on a track day. Same goes for the master cylinder. I’ve seen brakes get so hot it melts the wheel bearing grease. The internal liner of the brake hose can also separate from the heat and intermittently restrict the flow of brake fluid. The brake line may need to be replaced in extreme cases.
if you have ABS, the pump may suffer damage. It’s hard to say for sure. If ABS brakes feel like they still have air in them, one trick is to activate the pump while back bleeding them. An air bubble gets in the ABS pump and normal bleeding won’t remove it. Rapid pulsating the pump with the system test tool will shake the air bubble loose.

mike_delphin

· #3 · 2 mo ago

This just happened to me a week and half ago on my 08 Sport Classic. My rear brake locked up on me while slowly cruising down the street. I was right at a traffic light and had to get out of the way for cars to pass. So the only thing I could do was rev the engine and drag the bike a few feet to cross the road to a parking lot. While doing this, I could see some smoke and saw I had caused a couple of skid marks on the road. Had the bike towed home and haven't had the chance to touch it yet.

The week before I found that I had a leak coming from the rear fluid tank (Rizoma Next tank) with a clear line. Before that leak all was good for years. So I bled the system and checked the tank and tightened the screws to make sure it was snug. That did the trick, so I then replaced the clear tubing with a new one (the old one was too long). Put it all together and all was well.

You mentioned that there should be some sort of vent on the fluid tank but I can't recall seeing one. There is a rubber bladder that goes under the top cover though. I think that in my case I don't have enough free play. When depressing the rear brake pedal, it's fairly easy to do but might not have enough distance (free play).

In any case, I managed to get a used replacement rear caliper from a friend who has the same bike. I'm not sure if the rotor and master cylinder are no good anymore but plan to have them replaced anyways.

Any advice?

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Monster4Lee

· #4 · 2 mo ago

Never had this issue. I must not brake hard enough.

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BOTT

· #5 · 2 mo ago

That's more neglect of changing out the fluid. Buildup under the quad seal cannot return to square. The quad pulls the piston back. This unloads the pads off the disc.

Push pads back into the caliper. This shows a freewheeling wheel. Pump the brakes back up. Wheels drag? Should spin as if pads retracted.

Last WSB race this year, #19 thru his duc in the dirt, he gets up, you could see the front wheel still spinning. Hot enough?

TUNA

Discussion Starter · #6 · 2 mo ago

mike_delphin said:
This just happened to me a week and half ago on my 08 Sport Classic. My rear brake locked up on me while slowly cruising down the street. I was right at a traffic light and had to get out of the way for cars to pass. So the only thing I could do was rev the engine and drag the bike a few feet to cross the road to a parking lot. While doing this, I could see some smoke and saw I had caused a couple of skid marks on the road. Had the bike towed home and haven't had the chance to touch it yet.

The week before I found that I had a leak coming from the rear fluid tank (Rizoma Next tank) with a clear line. Before that leak all was good for years. So I bled the system and checked the tank and tightened the screws to make sure it was snug. That did the trick, so I then replaced the clear tubing with a new one (the old one was too long). Put it all together and all was well.

You mentioned that there should be some sort of vent on the fluid tank but I can't recall seeing one. There is a rubber bladder that goes under the top cover though. I think that in my case I don't have enough free play. When depressing the rear brake pedal, it's fairly easy to do but might not have enough distance (free play).

In any case, I managed to get a used replacement rear caliper from a friend who has the same bike. I'm not sure if the rotor and master cylinder are no good anymore but plan to have them replaced anyways.

Any advice?

To confirm the reservoir is vented, remove it and blow air into the hose pipe. If you can’t blow air into the reservoir, it’s not venting. When the fluid gets hot it expands and should push the small amount of air out the vent in the reservoir. If the air can’t vent, it pushes the fluid into the brake caliper.
you definitely need free play at the master cylinder, but even if the brakes drag and overheat the caliper, it shouldn’t lock up. It locks up because the air under the cap can’t escape.

wdietz186

· #7 · 2 mo ago

Usually, the cap will vent through a tiny hole in the cap or a slot up the inside of the cap where it threads onto the reservoir. The vent doesn't have to be very big because the fluid displaced in operation is tiny. But it does have to be there to compensate for expansion of the fluid especially if the reservoir is kept full.

BOTT

· #8 · 2 mo ago

Cap should have a cut line inside the cap. Might even cut thru the threads of the cap. If say that is clogged, diaphragm would not breath the back and forth of atmosphere, keeping the air pocket from moving. The diaphragm follows the vac on the other side [not cap's side] and expands itself down the master's reservoir with pad wear.

1duckyboy

· #9 · 2 mo ago

Make sure you have adequate free play in the brake pedal or when the fluid heats up and expands it will drag and eventually lock up the rear brake.

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EastBayRider

· #10 · 2 mo ago

Thanks for this (including the Castrol React suggestion). Tiny vents can make a huge difference. Happened to me last summer. Front wheel locked up when I was rolling to a stop, bike threw me off, rather very unexpectedly. I replaced the fluids, bled the lines, so it shouldn't happen again --- knock on wood.

TUNA

Discussion Starter · #11 · 2 mo ago

As a follow up, I searched the Brembo technical web pages. I found one mention of the venting issue. Brembo says the system must have a 1mm vent hole. Makes perfect sense. But when I blow into the bottom of their reservoir it’s air tight. Some systems have enough room that the air beneath the cap can’t get hot enough to expand enough to push the brake fluid hard enough to apply the brakes. If you were paying attention in your High School physics class, you’ll remember hot air in a confined space can push a liquid, brake fluid. In the right circumstances the hot air below the cap can and will apply the brakes.
I took the rubber diaphragm out and used a red hot needle to melt a tiny hole in the low point of one of the folds. That did the trick. Air freely vents when I blow into the reservoir. The caps have a slot cut through the threads. Brembo obviously intended that to act as a vent. Even the plastic ring that sets between the diaphragm and the cap has vent groves in the plastic. There just needs to be a tiny hole in the diaphragm. I’ll bet you never dreamed your High School Physics would be useful one day.

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DaveAu

· #12 · 2 mo ago

Monster4Lee said:
Never had this issue. I must not brake hard enough.

Me neither, I change fluids every 2 yrs riding all year round, so no corrosion or sediment build up from moisture contamination or lack of use like in a winter layover in the northern hemisphere.

Maintaining brakes seems simple, but it's easy to get it wrong and not do a good enough job. The correct spec fluid for the system is vital. All levers must be nice and firm when pulled on, not soft/ spongy. If spongy, there are airbubbles trapped in lines/ calipers just waiting to heat up and lock the brakes on

. Poorly maintained and very old systems can have blocked vents and channels or leaky seals letting air in so need a full overhaul to restore peak performance.

Brakes.....gotta have good brakes.... correct maintenance is vital

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Monster4Lee

· #13 · 2 mo ago

I change fluids too, that is a good idea.
The whole northern hemisphere is not a frozen white wasteland in winter however.

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BOTT

· #14 · 2 mo ago

Better put a diaper over the master. Any pin hole to the diaphragm is going to find the shortest path like the Big E.

You're killing me Larry! HA, let me redesign the ATMO path.

Chuckracer

· #15 · 2 mo ago

NOT filling your reservoirs to the brim helps as well.

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