Check your master. There is a residual check valve that keeps static pressure on your rear brakes, it keeps a static pressure on the wheel cylinder to allow front disc and rear drum brakes to come at the same time by not having to overcome the rear spring pressure. No static pressure allows rear cylinders to collapse allowing air to seep in.

Is there a chance you could’ve gotten air in the ABS lines? It might be a good idea to take the car to a shop to have them bleed the entire system (ABS included). Bleeding ABS requires a special tool that puts the ABS system in a bleed state. This is not possible for a home mechanic without a professional scan tool.

I wonder if you have a bad brake hose that is preventing fluid from returning back up toward the master cylinder. That could potentially make it so the ABS can’t do its job, and one or more calipers would just lock as you’ve described.

Otherwise, I’m not really sure what’s going on here. What was the initial problem you were trying to solve by replacing the brake booster?

However, looking back I should’ve provided a little more clarification because it is correct that a traditional vacuum brake booster can’t lead to spongy brakes and the current explanation isn’t all that clear on it.

Hydro-brake boosters can suffer from two problems that an amateur could diagnose as spongy brakes, although a professional mechanic would likely only diagnose one of the conditions as “spongy” and it’s typically pretty easy to rectify.

First, there can be air in the system. This is more common after replacing any components in the power steering system – including the hydro-brake booster itself. Compared to bleeding brakes getting the air out of the power steering system is generally pretty easy, even if it can be a bit more time-consuming.

Second, there could be a leak in the hydro brake booster. This isn’t going to lead to true “spongy” brakes, but it will lead to a drop in brake pressure as you hit the pedal, especially if you hold the pedal down. I think this is more what I was trying to get at without diving too far into the nitty gritty details of each system. An amateur could very easily mistake this for spongy brakes.

There’s no doubt that if your vehicle has spongy brakes, 99.9 percent of the time it’s because there’s air in the system. Even every other “cause” I listed simply introduces air into the system one way or another – which creates the spongy brakes.

When the booster is not providing enough assist the pedal will actually start to feel much harder than before because with less boost the same pedal pressure produces less pedal travel.

I am a mechanic and can’t think of a scenario where it could cause a spongy pedal. If anyone could enlighten me, please do so because I would hate to be wrong and not know it.

I guess the next thing I would check would be the rubber brake hoses. Make sure none of them are swelling or doing anything strange when you press on the brakes.

I would also double check that the brakes were bled properly and that there is no air in the lines, especially if you have ABS. Air can get trapped inside the ABS module and often requires a professional to properly bleed.

It’s possible. That’d be high on the list of things I’d try next.