Pronunciation of zebra, Libra, apron, April, maple, etc.

There is a tendency to use historically short vowels before consonant clusters thank to pre-cluster shortening: for instance, Popsicle vs icicle 


pr: April, apron
br: hubris, zebra, Libra
tr:  patron, matron, crater, gater, meter, writer, motor, peter, Peter, titre, tutor, liter (vs litter)
dr: cathedral, cider, cadre, cedar, glider, radar, rider, spider, trader, wader, odor, nadir
kr: acre, lucre
gr: tiger, ogre, pager, wager

pl: maple, staple, scruple, tuple, disciple (vs couple), pupil
bl: able, fable, noble, ruble, stable, table, cable, libel, label

tl: title, beetle, tootle, fatal, hotel, total, betel, brutal, vital, fetal, natal, petal (vs. metal)

dl: bridle, cradle, idle, sidle, needle, poodle, modal,(compare huddle, saddle, hurdle, middle, -ddle)
kl: cycle, icicle, fecal, focal, vocal

pn: open, ripen,