The Tang monk swept one storey with his broom, and then went up another.

帚子 lets its second syllable go light, zi, against 學子 and 弟子 elsewhere in the chapter, which keep the full zǐ. A broom-handle is not a scholar. Both 一 here come out as yì before the rising tone of 層.

Sweeping on in this way he came to the seventh storey, and by then it was already the second watch.

二更 is the second of the five night-watches, roughly nine to eleven, and 更 takes gēng in that sense rather than the gèng of comparison. The same word turns up shortly as 三更 with nothing different about it but the numeral in front.

The elder was growing weary, and Pilgrim said, "You are worn out. Sit down a while and let old Sun sweep in your stead."

掃罷 ends on the soft ba that turns an instruction into a suggestion. The same character was the verb bà twice in the passages just before — 耕犁罷短繩 and 祝罷 — and the reading turns entirely on where in the clause it sits.

Tripitaka said, "How many storeys has this pagoda?"

The question is asked at the seventh storey, and the answer decides how much of the night is left. 層數 is the count of storeys, with 數 in its noun reading shù.

Pilgrim said, "I dare say there are thirteen."

怕不有 is an everyday hedge that means the opposite of what it looks like. It is not a doubt but a confident estimate — I shouldn't wonder if there aren't thirteen. 哩 keeps the light li it takes throughout this project.

The elder, bearing up under his weariness, said, "It must certainly be swept. Only so is the vow satisfied."

耽著 takes 著 as zhe, the one that means while it is going on: he is carrying the tiredness as he speaks. 本願 is the original vow, the one made at the Temple of the Dharma Gate and recalled earlier this same night, and 方趁 makes the sweeping its condition — only so is it met.