Hydro Boost and Rear Disc Brakes

I’ve been down the rabbit-hole the last few days looking up information about Hydro Boost and how it performs on our trucks when replacing the rear drum brakes with discs.

The part numbers look to be identical between my own HBB and one for a Land Cruiser, or 4Runner, which both utilize rear discs. That tells me that I should be able to use my current MC/HBB to power rear discs. (If service manuals are to be believed.)

Obviously drums and discs have different requirements for volume and pressure. If the difference isn’t made in the MC or HBB, then where is it made? The lines appear to be the same size. I would think the difference would be in the Housing where the lines attach.

If someone out there is knowledgeable enough to take me to school on this, please come pick me up!

Anyone with Hydroboost/rear disc swap experience, I’d like to hear from you, too!

 

uh, are you confusing the booster with the master cylinder? because it's the master cylinder that you need to look at, not the size of the brake lines. almost everything uses one of maybe three sizes of brake line.

 

Yes and no... The MC body is part of an assembly that is called the “Hydraulic Brake Booster” in the factory service manual. To make things even more confusing, the Tacoma uses the same part number for this assembly as vehicles using disk brakes on the rear:
So if the larger HBB has the same part number, I would assume that the Master Cylinder bodies do too, right? Maybe that’s a bad assumption?

 

@stevotivo12
https://www.tacomaworld.com/threads/big-ole-brake-upgrade-thread-for-third-gens.525568/

 

^I read through that thread, but he has a Sport, so he should have a vacuum booster. I suppose I could drop a question in that thread to keep everything together though. Thanks.

 

Is your concern just volume of fluid?
If so, I don't think you have any worries. Regardless of disc or drum, the volume of fluid difference is going to be accounted for on initial fill and bleed. For example, let's say the original drum took X ounces from everything dry to completely full. That's master, lines and actuators. Now you put on discs and it takes X+1 ounces. That one ounce will be accounted for on the initial fill. Remember, once you brake circuit is full, there is really no flow in the lines. You are just moving the fluid differential between the rod and base side of the piston.

 

Proportioning valve is what you need to look into

 

I’m more concerned with stroke volume and line pressure.

I read that hydro boosted trucks operate with double the line pressure of vacuum boosted trucks, but drums don’t require much pressure. I’m not sure How Toyota accounted for the difference. If I install rear discs - will they see the necessary pressure to operate well?

If not, is my brake pedal going to be on the floor all the time?

 

Depends on the size of the pistons in your replacement calipers, there is no simple answer. If you want discs you gotta come to grips with the fact that you're spending a lot of money to make your brakes worse so that you can look cooler. There are very few legitimate use-cases for this "upgrade" on these trucks.

 

The V8 4Runner had hydro boost brakes too. Adjusting your brake bias is done via the proportioning valve IIRC

 

Wut? Towing would be one... much better heat dissipation.
Only reason Tacos didn't get discs was cost

 

If that was your use case why would you not upgrade the brakes that do 75% of the work instead? And if you're pulling a trailer, why are you bombing-ass downhill so much that you're fading your brakes in the first place, let alone your REAR brakes?

ugh I gotta stop coming into these threads

 

I don’t want this to turn into the normal disc vs drum shit-show that is all too common on this type of thread.

Make whatever assumption you want about “why, right, wrong” and focus on the “how.”

Proportioning valves don’t proportion anything or adjust bias. They limit pressure seen on rear drums to prevent them from locking up (because they tend to lock up at relatively low pressures.)

With that in mind, I wonder if there is one installed in the rear brake lines that I could pull to achieve higher pressures for the rear corners...

 

Not exactly true. The factory proportioning valve doesn't let you adjust anything so you replace it with an adjustable one so you can have adequate pressure for the rear discs and that should be he end of it

Edit: in theory anyway... I don't know if there are bolt on options available to purchase

 

My understanding of proportioning valves is that they are full-flow valves up to somewhere around 250psi, where a drum normally tends to lock-up, then the valve cuts the flow down in order to prevent that from happening.

With rear discs, that pressure can get a lot higher before you lock them up, so you want full flow (especially with ABS.)

 

Not sure but that part is where you will want to adjust so that your new rear discs actually improve braking and aren't just useless bling

Edit: found this online
https://www.wilwood.com/mastercylinders/mastercylinderprod?itemno=260-11179

 

@stevotivo12

 

I don't think that's right because that's not bias
It goes x psi to front and x psi times bias (say 50%) to the rear.

 

Necro thread.

Thanks for asking OP I’m looking into this as well. The proportioning valve won’t work I don’t think because everything is controlled with a combination of the hydro boost and wheel sensors.

What I’m curious if the hydro boost is capable of creating enough pressure to run upgraded brakes.

The 4runners have bigger front brakes and run rear disks. The Land cruiser has even bigger front brakes. So if the parts are the same I would assume so?

 

I would assume the other vehicles booster is setup differently. These things are not cheap to replace so its not a common option. I have 5th gen 4runner brakes and currently have a rear disc setup sitting in the garage. I have read back and forth all the shit on but screw it going to do it anyway. I have hit the wall as I did not take into account the electric hydro booster. If you have a sports with the simpler one its easy and swap out to the sequoia.

I did email SOS performance about it and they said it would work but would have a softer pedal feel with the e booster. Now how soft I am not sure and what concerned me which has left me not installing it "yet".

If anyone has any insight I am also interested. I am tempted to be the guinea pig and go for it but its not an easy reversal since you cut off the drums.