Air in my brake lines

[SpencerPJ]

So, I have an 83, 51k miles.  Rear brakes, bled the notorious high spot under the handle bars, perfect, air gone, perfect brakes again.  A week later, 300 miles, and they are going soft again.  Brake fluid is fine, no visible leaks anywhere. How can air get into system? I always clean up pistons when I do a brake job, never have seen pitting, they are remarkably clean.  It has been a good while since I've done a flush / fluid exchange. Any thoughts are appreciated, thx. 

[Freebird]

I would start by bleeding them again.  Could well have been an air pocket somewhere. But yes, if it’s been a long time since a flush and change, I would do that.  

[SpencerPJ]

1 hour ago, Freebird said:

I would start by bleeding them again.  Could well have been an air pocket somewhere. But yes, if it’s been a long time since a flush and change, I would do that.  

I have a mity vac, any problems sucking it through the system, FL then RR keeping the rear reservoir filled?  I thought I read once that you can suck crap into your system this way, should I first check / clean Rear reservoir? thx

[Squidley]

I have had luck with reverse bleeding hydraulic systems on the bike. I use horse syringe and attach it to the bleeder and push it through the lines into the master cylinder. 

[skydoc_17]

Hey Spencer,

Because the rear master cylinder reservoir is vented, if the seal set in the master cylinder is compromised, it will introduce air into the rear/left front part of the brake system. A rebuild would solve this issue. While you are at it, look on Ebay for a MKII  Rear master cylinder, do the rebuild on that, and just swap out the MKI rear master for the MKII rear master. The MKI rear master is slightly undersized to push fluid to two brake calipers in my opinion, and the MKII rear master is an excellent upgrade. The outside of the body casting is identical between the two masters, it's the inside bore that is enlarged on the MKII master. I have also seen the crush washer on the proportioning valve leak as well. I would tighten EVERY connection in the rear braking system before moving forward with the rear master rebuild. Hope this helps,

Earl

[larrydr]

I have a question only ...Is there a O-ring leaking and sucking air ....That is what I found on my 83 ZVX1200 ...I noticed a wet spot on the front under the rubber

[skydoc_17]

Hey Larry,

As I said, the cap to the master cylinder IS vented. It's not the job of that O'Ring to seal the master cylinder from the atmosphere. If you are seeing leakage at the vent holes in the cap then the seal in the master cylinder is beginning to fail. A rebuild of that master cylinder will stop the leakage. 

Earl

[SpencerPJ]

10 hours ago, skydoc_17 said:

Hey Spencer,

Because the rear master cylinder reservoir is vented, if the seal set in the master cylinder is compromised, it will introduce air into the rear/left front part of the brake system. A rebuild would solve this issue. While you are at it, look on Ebay for a MKII  Rear master cylinder, do the rebuild on that, and just swap out the MKI rear master for the MKII rear master. The MKI rear master is slightly undersized to push fluid to two brake calipers in my opinion, and the MKII rear master is an excellent upgrade. The outside of the body casting is identical between the two masters, it's the inside bore that is enlarged on the MKII master. I have also seen the crush washer on the proportioning valve leak as well. I would tighten EVERY connection in the rear braking system before moving forward with the rear master rebuild. Hope this helps,

Earl

Thanks Earl, Last time I was in the rear master cylinder, topping off, I dropped that stupid washer around the bolt, and replaced it with a rubber washer.  Could that cause issue?  It seems sealed and does not leak there.  I did notice the inner rubber was collapsed down from the lid.  Should I actually remove lid and investigate?

[saddlebum]

10 hours ago, skydoc_17 said:

Hey Spencer,

Because the rear master cylinder reservoir is vented, if the seal set in the master cylinder is compromised, it will introduce air into the rear/left front part of the brake system. A rebuild would solve this issue. While you are at it, look on Ebay for a MKII  Rear master cylinder, do the rebuild on that, and just swap out the MKI rear master for the MKII rear master. The MKI rear master is slightly undersized to push fluid to two brake calipers in my opinion, and the MKII rear master is an excellent upgrade. The outside of the body casting is identical between the two masters, it's the inside bore that is enlarged on the MKII master. I have also seen the crush washer on the proportioning valve leak as well. I would tighten EVERY connection in the rear braking system before moving forward with the rear master rebuild. Hope this helps,

Earl

Given what you just stated about the difference between the MKI and the MKII rear master cylinders. Would the reverse not be true if you have de-linked the brakes and it not then be beneficial to install a MKI rear master cylinder onto a MKII? In my mind the slightly smaller piston diameter of the MKI rear master would increase the application pressure applied by the rear caliper over that applied with the MKII. As it stands I feel I could use a bit more braking force on my rear caliper than I currently have. Of course I get that for some that could make the rear brake too aggressive, specially on slippery road surfaces. 

[skydoc_17]

Larry, there is an O'Ring (so to speak) in the cap to the master cylinder reservoir. Because the Reservoir is VENTED Spencer, it doesn't matter weather you use a washer or an O'Ring  to seal the fill port. ONLY the fluid from the seal on the plunger of the master cylinder puts pressure on the fluid going to the caliper. If you bleed the brakes and air returns, it can ONLY be coming in thru the seal of the master cylinder or the caliper seals. (As long as ALL of the banjo bolts are tight) You can drill holes in the cap of the reservoir and it WILL NOT let air into the brake system! Period. If you see fluid coming out of the vent holes in the reservoir cap then the seal in the master cylinder is failing and allowing pressure that should be going to the caliper to escape back into the reservoir, pushing fluid out of the VENT holes. 

Ben, I'm pretty sure you have De-Linked your brakes. So you are telling me that you took a master cylinder that was powering TWO 4 piston calipers and are now powering ONE 4 piston caliper and you wish you had more braking force? If this is correct then you have another issue like sticking pistons in the rear caliper or you may want to select another type of brake pad my friend! You are dealing with two forces between the MKI and MKII master cylinders, Volume and pressure. There is MORE than enough volume in the MKII master cylinder to lock up the rear caliper. Both my 87'VR and my 89'VR are De-Linked, and I had to go to the Kevlar rear pads to control the lock up issue. Excessive PRESSURE prematurely wears out the seals on the master cylinder and calipers. Volume on the other hand, gives you plenty of braking power WITHOUT the damage to the seals, and I control the lock up issue with brake pad selection. Because we are talking about an 83'VR here, the ONLY item that can be used from the MKII Brake system is the MKII rear master cylinder. NONE of the calipers from the MKII can be swapped onto an MKI without mods done to the lower front forks, and the rear rotor and caliper mounting bracket.   

I hope this clears this up for you.

Earl

[larrydr]

As we are talking about brakes and we all need good ones ..

A rider here lost his life here on a down hill curve  in June 2021 ... I do not know any details  than just that ..I have been on that same road and the same curve where  this accident happened , and it is is  a short down hill  curve , you be have to be in full control of your bike .. I had to drop my speed a least in by half  the bike or  will go into a slide depending on how you apply your brakes ..When road stays a bit wet , it is not a good corner

[saddlebum]

10 hours ago, skydoc_17 said:

Ben, I'm pretty sure you have De-Linked your brakes. So you are telling me that you took a master cylinder that was powering TWO 4 piston calipers and are now powering ONE 4 piston caliper and you wish you had more braking force? If this is correct then you have another issue like sticking pistons in the rear caliper or you may want to select another type of brake pad my friend! You are dealing with two forces between the MKI and MKII master cylinders, Volume and pressure. There is MORE than enough volume in the MKII master cylinder to lock up the rear caliper. Both my 87'VR and my 89'VR are De-Linked, and I had to go to the Kevlar rear pads to control the lock up issue. Excessive PRESSURE prematurely wears out the seals on the master cylinder and calipers. Volume on the other hand, gives you plenty of braking power WITHOUT the damage to the seals, and I control the lock up issue with brake pad selection. Because we are talking about an 83'VR here, the ONLY item that can be used from the MKII Brake system is the MKII rear master cylinder. NONE of the calipers from the MKII can be swapped onto an MKI without mods done to the lower front forks, and the rear rotor and caliper mounting bracket.   

Hi Earl I am not saying I have an issue with the rear brake and I totally get what your saying. Both my master and my rear caliper are in good shape and my wheel will lock up on loose gravel and will slow me down on asphalt. In essence all I am  saying is I would like to increase the stopping force of my  rear brake, more of a personal thing really. BTW when I de-linked I completely eliminated the proportioning valve from the master and connected the line directly to the master. My thoughts here was simply to increase the force a bit and given that the seals on the caliper are no different than front ones I imagine the minimal increase should not hurt them. So in other words I would trade off some delivery volume for increased delivery pressure, given the ratio difference increase between the master piston diameter/face area and the total face area of the combined  caliper pistons which of course would also increase pedal travel.  As I said this is more of a thought process than an actual problem.

[djh3]

I had an "Oh $hit" moment a few years back on the RSV. Never had any real problems except the occasional rear lock up. While in the mountains following a SUV he was on the brakes so much I kept catching him. Mine got hot and boiled the fluid. Scared the bejesus out of me. So pedal went all spongy like had air. I used the mighty vac and emptied reservoir then ran probably a half bottle of DOT 4 thru it.

[saddlebum]

10 hours ago, djh3 said:

I had an "Oh $hit" moment a few years back on the RSV. Never had any real problems except the occasional rear lock up. While in the mountains following a SUV he was on the brakes so much I kept catching him. Mine got hot and boiled the fluid. Scared the bejesus out of me. So pedal went all spongy like had air. I used the mighty vac and emptied reservoir then ran probably a half bottle of DOT 4 thru it.

I have been trying out the bosch ESI6 brake fluid and so far like it very much. I find improved clutch and brake response over the dot 3 and dot 4. It is a little more expensive though https://www.boschautoparts.ca/en/auto/brakes/ca-esi6-brake-fluid

here is an interesting comparison chart of brake fluids which includes both  wet and dry boiling points. For those who don't know the difference, The Dry Boiling Point of brake fluid refers to the boiling temperature of fresh, new brake fluid from an unopened container. Whereas the Wet Boiling Point is defined as the temperature DOT brake fluid will begin to boil after it has absorbed 3.7% water by volume. DOT brake fluid will reach this level of water volume after roughly 2 years of service, which is why it is advisable to renew your brake fluid every 1-2 years. https://www.lelandwest.com/brake-fluid-comparison-chart.cfm