2018 Prius Brake Booster Master Cylinder

probably not, at 70k, but call toyota customer service, and ask for goodwill warranty help.
that is a very early failure, and has to be defective.
i would also ask for a guaranty that it will solve the problem. sounds like the dealership is not sure

 

A blown 120 ampere DC/DC link in the fusible link block assembly (seen in my previous posting) and a blown 30 ampere ABS-MAIN or ABS MTR NO.2 fuse would be consistent with a short circuit in the brake system. The Electrical Wiring Diagram (more info) shows that those two ABS fuses don’t serve any other loads, though a fault in the wiring, connector, or junction block could have similar effects.

There’s no part called the “brake booster actuator”; rather, the three major assemblies in the electronically controlled brake system are:

• The brake booster with master cylinder assembly, part number 47050-47551, list price $1,405.84 (shown in catalog Figure 47-02, Brake Master Cylinder);
• The brake booster pump assembly, 47070-47090, $569.60 (in Figure 47-08, Brake Tube & Clamp); and
• The brake actuator assembly, 44050-47060, $752.08 (also in Figure 47-08, Brake Tube & Clamp).

You can read all about each assembly and what it does in New Car Features (more info). Each assembly is sold only as a complete unit; the Repair Manual doesn’t describe any internal repairs or adjustments.

The problem is to be sure which of these parts (if any) has a fault, and to be sure that the trouble isn’t actually in a wire or connector. If it were my car, before I’d authorize any of the expensive assemblies to be replaced, especially if the dealer isn’t completely sure of the diagnosis, I’d ask them to explain their troubleshooting process in detail, and I’d check their work against the Repair Manual (more info) procedures. Did the dealer scan for diagnostic trouble codes, and which ones did they find? Exactly where did they make electrical measurements, under what conditions, and what were the readings?

As described in the Warranty & Maintenance Guide (PDF), since the basic warranty has expired, and the brake system isn’t covered under the hybrid system, emission control, or other warranties, Toyota wouldn’t have an obligation to pay for the repairs.

That said, as @bisco kindly suggested, you could ask Toyota for goodwill consideration. “Goodwill,” in the industry jargon, is when an automaker offers to pay some or all of the cost of a repair that wouldn’t otherwise be covered under any warranty, in the interest of good relations with its customers. It’s a fair request here: the major assemblies of the brake system should last for much longer than 70,000 miles, and a premature failure, unless there was some outside influence, suggests a latent defect.

I think @bisco's idea is that the dealer should share the risk of their diagnosis being wrong. If putting in one expensive part doesn’t fix the problem, or worse yet, also causes the new part to be damaged, you’d want the dealer to pay any further costs to complete the repairs. I’m not sure this is something a dealer would agree to, however.

 

I just mean, what if they replace it, and the problem doesn’t go away

 

My first thought was rodent damage. Is the car parked outside? Or could it be water/flooding damage?